Depends on what you'er doing.
Personally for gaming at that res I'd be looking at a 4850 1gb or 9800gtx+ 1gb minimum.
and ideally looking at a gtx260 or 4870 1gb (£170-190) so nearly twice the budget
If all you're doing is internet browsing, office work and watching dvd's then you should have no real problems with your current card.
esp with xp, all you need to run a desktop at 1900x1200 32bit is a card with a little bit of processing power and 128mb of memory
Swap to vista and you need a chunk more gpu power and memory for some of the fancy effects. (but a 8600gt 256mb should be fine)
basically graphics card requirements breaks down to
desktop = very little
hd video decodeing = a bit
simple 3d apps = a bit
games = a lot
PS please don't get a 9600GSO they are really quite crappy, a 9600gt is a better card (but I'd not game at that res on a 9600gt)