what graphics card are you thinking of upgrading too. you will find that your present card is just as powerful or better than most of the new budget/mid range cards available. the only reason to upgrade is to use dx9 at its full potential as your old g/card is not dx9 compatable.
heres link to see if its worth upgrading click here
the fx6800 or the new fx6600 will be worthy upgrades to your old ti4200 but you will need to check that it will run on your mobo at x4 agp. as jamiebrk has pointed out his new radeon will not run on a 4x mobo but have not heard of geforce agp x8 fx graphics cards not running on mobo's at x4.
maybe someone else will be able to shed more light on the subject for you.
also it would be worth upgrading ya mobo, as it would be a waste of power running a 6800 at x4 rather than its proper x8 speed. can recommend the ASUS A7N8X-E Deluxe. excellent mobo if you're into overclocking or not, packed with lots of features and can use all athlon xp CPU's.
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