I think a 1gb 6850 is your best bet.
It's a good bit faster than a 550ti, especially at resolutions above 1680 x 1050 and equally important... it draws about the same power as your 8800GTX (slightly less) and also only uses 1 6-pin PCI-E connector.
So if your 8800GTX runs stable under max load so should the 6850.
Here's an online shop which has a 83£ deal on a MSI 1GB 6850 OC
http://www.overclockers.co.uk/search...d=&search=6850
I've dug a bit and seems there are people who have plugged a quadcore q6600 or q6700 (higher ones won't work) into their dimension 9200, but they aren't officially supported.
You will be cutting it very close PSU-wise though since the C2D has a TDP of 65 Watt ,while the C2Q is 105W..combine this with a graphics card which draws quite a bit of power, compared to your PSU rating, and you could well run into trouble.
Dell generally rate their PSU's pretty well, but if you need to upgrade the GPU,CPU and PSU you might aswell save a while longer and build a new budget PC.
edit : I'd forget about the 640..it seems to perform pretty bad from the one review i can find and is mostly intended for HTPC's or mini PC's
http://www.tomshardware.com/reviews/...ew,3214-3.html
That is if a 6850 physically fits in your Dell, which is something I didn't look into!