Just done that - 256 to 768MB. There was some increase in speed although I have noticed that editing is far easier with less disc activity. Not captured or rendered as yet.
Depending on your motherboard bios, if you up the FSB speed, your XP1700 may already be capable of running at XP2100 speed. If you can increase FSB in the bios, increase it maybe 5mhz at a time, then see if everything works. DO NOT increase it beyond 155-160 FSB, as this may stress out your other hardware, or corrupt your drives. Below this, there shouldn't be any problems. If you dont want to do this, consider moving to the XP2200, as it has an improved core and runs cooler, at least according to some posts I've seen on overclocking sites...