Unless I am wrong and you are not on a PC100/133 i845 motherboard, definately don't upgrade to the top video card, and don't BUY 1GB of PC133 even if your board supports it. PC133 doesn't offer the P4 enough memory bandwidth, not to mention it is very expensive now. You won't be happy with the results of that expensive RAM/ GPU upgrade. All that upgrade money could be put to much better use upgrading other key components. If all this is overbudget, I'd say downgrade the video card to a R9800 pro and pair it with an NF7-S/XP2500+ or mobile XP2500+ and some PC3200. If you truely want an X800 or GF6800, upgrade your system and pair it with a nice Athlon 64.
ABIT IS7, P4 2.6C, 512MB Corsair TwinX PC3200LL, Radeon 9800 Pro, Santa Cruz, TruePower 430watt