What is a Radeon 7800 Pro? Do you mean 9800 Pro? if so, it's better than mine. Another 512MB of RAM would be a good start, and if you do want to upgrade your graphics, a nVidia 7800GS would be the best bet, imo.
I would go with claytons advice. It really depends on what you play. Any game up to 6 months or so ago will probably run ok, you may struggle a bit with the latest ones perhaps like FEAR. You have a choice to make, you could keep what you have add more memory and perhaps upgrade the graphics to a 9800 Pro or even a GeForce 6600GT(I don't know if theres a significant difference between the 9700 and the 9800 but I'm sure someone will advise there) or you could put the money towards a new system. It depends really on what games you play. If your PC does all you ask of it just now I would put it off and wait. In a few months time you'll get more for your money.
I wouldn't bother going for a 9800 Pro or any 6600. Not much of an upgrade. Either a new motherboard etc., or the best AGP available (7800GS or possibly X850 XT PE) if you intend to stick with your current motherboard.
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