ATI Radeon X850XT Platinum. Upgrade?

Any more details?

Like: monitor resolution you play on and/or processor speed?

Esop!

might need to know ur cpu as well as it might bottleneck a high end gpu

Yeah, buy the most expensive card your budget can afford . Really without more information I can't make a better suggestion.

I *just* went from an X850XT to an HD 4670 today, and I've gotta say it's great. I'm replaying Oblivion on totally maxed settings, and nothing's ever looked better (to me). I'm playing at 1440x900, and so far it's running beautifully. Much quieter, too (I have the MSI version). I'm loving it. For ~ $80 it's a steal.

I forgot the budget.

Anything within reason.

So you have a Pentium EE 965 right? A Radeon 3870 or 4670 would be a good sub $100 upgrade for you. If you spend a little more money though you can go with an 8800GT 512. The Radeon 4850 is a good buy, but unless you plan to upgrade the rest of your system anytime soon you won't get much of a benefit out of anything faster than an 8800GT. If your system can handel that X850XT Platinum, then it should have no problems with the Radeon 4670. On the other hand a Radeon 4850 and 8800GT need more power,(even the 3870 needs a little more power) so make sure your PSU can handle it.

The 4670 would be your best bet both as far as being sure the PSU could handle it and from the view point of not wanting to put something in there that your CPU will restrict. A 3850 would be about the same performance wise but would need more power as would a 8800GT 512 which would be a bit better i guess. The 9600GSO is just a renamed 8800GS which is a cutdown 8800GT but would use slightly less power. A 4850 would most likley be restricted a fair bit but that just means the FPS will max out and you can run with more AA/AF.
I would look on it as a price performance exercise.
You would need to check your PSU first but Dell PSU's are generally pretty good.
The 4670/3850/9600GSO would be a good fit with your machine power and performance wise
The 8800GT512/3870 would probably be ok but i would start wondering about restrictions here.
And im sure a 4850/9800 would definatly be restricted.
So have a look see where the prices are and go from there.
Mactronix

Well, my X850XT has a six pin power connector (I think that's what it is), an extra power connection at least, so doesn't that mean the HD 4670 *definitely* takes less power? I suppose the old connector could be for safety, but, I'm thoroughly impressed with (my) HD 4670. HDR is in fact the coolest thing.

4850's are down below $140 with rebates

You forgot to tell the Op to be sure to uninstall the old drivers properly first.
Mactronix