thank you frass, May be I won't need an SDI because there'll be another broadcasting server that needs only a DV input (it's outputfrom VT) & this is available i think......
I noticed that you didn't say anything about the other systems.....why?

Take what I say with a pinch. I've been contributing to the various VT forums for a couple of years.

DigiSuite, Velocity, Targa 3000 all have there place and values particularly with a supporting nearby dealer.

A vanilla VT gives you DV input (with audio), component YsUV (but not RGsB), Y/C and composite input. Analog audio is high impedance line level. Output is the same but you don't get DV out and possibly you won't ever. Apart from the luxury of multiple DV input devices into the switcher, and native DV timeline support (powered by MainConcept DV codec, which isn't bad and is possibly a distant relative of Vegas').

The aim of the VT game is to be realtime all of the time. Doesn't mean it will be. This equates to each new (annual so far) release of VT having a newer minimum spec, but the hardware doesn't change. Less waste! Standard def isn't changing and HiDef hasn't proven itself in HDCAM or HDV/D-VHS formats yet.

The aforementioned non-VT cards are technology locked. So expect new cards with new softwre features. Or a predominant use of expansion plug-ins. NewTek almost have an Atari outlook for "Power without the Price"!

You can use VT with a 32bit PCI system, a PIV or fast AthlonXP and IDE drives. You are just less likely to obtain realtime. This means you wait for a background rendering light to come on before you can playback cleanly. Question comes as to why did you want the VT in the first place.

USPs:
Switcher and multi-tap DDR (source and recorders).
3D DVE construction kit (requires LightWave skills and time)
2D DVE kit (Aura2 skills but pretty straightforward for wipes)
BoB expands functionality to studio/event work or pro hook-ups.
SX8 BoB gives AES EBU audio in and out.
SDI option is reasonably priced (<$800)
YUV pipeline so 8bit is adequate where 10bit is needed for RGB.
Dealer support in North America and islands in the Pacific is exemplorary.
Chances of actually using an effect or lower third that isn't just a slice of cheese is significantly more likely in VT - once learned.
Oversampling (sub-pixel) and greater internal pixel dynamic range makes for a quality solution.

Downsides:
SDI is still waiting audio to be enabled.
YUV pipline so RGB sources (typ compositors/animators) need care when converting.
Audio filters and controls are nothing to write home about.
Audio is 48kHz 16bit stereo limited - other sound cards are not formally supported.
SX8 has had some variance in audio quality across the ports. An RMA programme should have sorted this. Consider this if buying second hand.
DVD authoring isn't attended to at all.
PAL support requires a wait for the software and a different batch of CDs for the DVEs, making dual standard support a bit of a hoop jump.
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Other bits:
Some of the owners actually make kit for Vt and the industry in general. e.g. Alternative AES/EBU (non switcher) BoB. A camera attached DDR that can record MJPEG 4:2:2 directly to HD from YUV/component and SVIDEO sources. (not out yet) So this type of kit can give you higher quality and better keys using your same awful DVCPro50, Pro25, HDCAM, DV, D8 etc gear, bypassing the tape and onboard compressors.

The Amiga was an exciting machine. NewTek's VT is something truly representative of 11 years on.

VT won't suit everyone. DV folk especially have to wrap their workflow around the power it provides. Whereas Vegas wraps itself around the operator. Both do uncompressed. VT does uncompressed better (YUV [UYVY, YUY2]).

Having fewer customers than Vegas. You can expect the chief technical officer to not only reply to your problems, but to try you with a patch. Rarely needed but this is how customer oriented they are.

The boss programmer is also British. So has fewer qualms about bypassing directx with his own custom code (hand optimised assembler).

Lastly Targa was soaked up by Pinnacle. DPS by Leitch. Play by GlobalStreams and even Matrox seem to be targeting volume DV whilst there is an advantage to using custom chips for DV.

VT won't solve world hunger. However its value proposition is so very broad. I encourage folk to route out a dealer and see a demo.

Vegas is a force in NLE land, don't get me wrong on that.

I had a brief look at your other topic, I understand you are looking to set up a studio for live webcasting, I don't think you need a very serious system in terms of video capability.

What you need to focus on is the success or otherwise of the webcasting component, from what I understand none of it is spectacularly successful. Certainly the industry seems to have gone prett cold on the idea. From memory the Sundance site has some good tips on video for webcasting. Given that your aim needs to be to reduce motion as much as possible I would think you could get by with just DV cameras and a analogue mixer. Composite video would be more than adequate. The other thing I guess you want is as little noise in the video as possible.

I'd just have a long hard look at your transmission medium side of things first, I suspect its going to be very limiting. I regularly convert streaming media at 320x240 30fps to DV for a friend to view. A simple cut after encoding becomes a 2 second transition as the entire image is rebuilt. These guys also feed the original broadcast via satellite so I guess its a good example of how good / bad realtime streaming is.