Your PC should be able to handle at least 20 tracks. The only thing that you are lacking on is the amount of RAM. 256K is not that high for a DAW. I would recommend a minimum of 512K. I'm using 1.1Gig and personally would not use anything less than that. If you're not using any plugins on those 20 tracks, then the amount of ram will be less critical.
The next thing you want to check is your hard drive performance. Most standard installed hard drive rotation rates are at 5400 RPM. You want at least a 7200 RPM drive for better track count performance.
The immediate thing you can check is to see if the hard drives "DMA" is enabled. This needs to be enabled and will usually boast your performance to twice of what it currently is, if it is not enabled. I don't know the steps off the top of my head to list them for you.
after looking at all the thing Red suggested, you might also look at grouping a number of tracks to buss type efx processing rather than have each track with a lot of different efx. cutting down on the number of individual processes going on.
...and after what PeterV suggested, you may want to look at some of the 20 tracks and see what you can mix down to one or two tracks.
I've done some projects where I was working on the track bed and got upwards of 20 tracks, but x-amount of it wasn't going to change (much) so I mix it down to one track.
I tend to keep multiple generational copies of my files too....so if I ever had to go back, I can do it with ease. So this way I take what was on 20 tracks and save it down to 2.