I agree with MOD and is exactly what I did with my system.
No point upgrading just one part of your system when you cpu will let you down.
Buy a 3800 or 4200 dual core
http://www.overclockers.co.uk/productlist.php?groupid=701&catid=6&subid=567
£40 or £70 depending which you go for but both overclock supremely well (my 4200+ runs at 2.75 ghz with no extra voltage).
sell your old one on the bay - i got £35 for my 3200+ - there are some mad people out there :d
this will make a huge difference to games and generally in xp full stop.
i went from an x800xt to a 8800 gts 320mb which was also a massive gain but from tom's hardware page you woudn't see anything like the boost i did.
i would stick with the x1900xt for now albeit add a 3rd party quiet cooler and you will be fine until you see what the new cards from nvidia are like at xmas.
the increase to a 8800 gts 320mb would be about 28% on average according to this:
http://www.tomshardware.co.uk/chart...2,441,444,445,449,451,450,453,448,447,455,454
But this will only apply if you upgrade your CPU as well.
If you really want all the eye candy now and those £250 notes are burning a hole in your pocket then upgrade the CPU which will cost you about £20 after selling your old one and buy the 8800 GTS 640 Mb with the rest of your money. You should still have change once you sell your x1900xt since they still make good money on the bay.
Take my four year old x800xt - some idiot bought it for £50
