Well you seem to be in luck for your CPU, your board is compatible with the 65 nm Core 2 Duos (e6000 series and earlier). I can only suggest that you shop around and try to find the fastest one you can get, though it may be quite expensive, and not worth the money in the grand scheme of things. If you can't find an official CPU compatibility list, contact Dell, and confirm that the CPU will work, and whether you will need a BIOS flash to get the new CPU to work.
A lot of the faster 65nm C2Ds can go for very high prices, as they are no longer made, and there is a greater demand from people hoping to extend the life of their old systems than there is supply for these discontinued chips. I think I recall the e6600 going for nearly $200 in some cases about a year ago. I doubt the prices on these have gotten any better since then.
Your PSU is enough to support a 5670 or 6670, but nothing higher than that. Do not expect great performance out of these games even with the upgrades, BF3 especially. If Battlefield Bad Company 2, and Call of Duty Black Ops are any indication, these games will not perform that well on dual core CPUs, especially older, slower ones like the 65nm Core 2 Duos. As far as the GPU goes, the 5670 will perform okay at lower resolutions, though don't expect great performance at high resolutions like 1080p in BF3, the 5670 is about equivalent to the 8800GT, which is listed as the minimum graphics card for that game. Modern Warfare 3 isn't likely to be as graphically demanding, and will likely perform better.
For the money required to locate parts compatible with ageing systems these days, it's often a far simpler, more cost effective, and rewarding spend, to simply look at buying a new pc.
Budget gaming pc's can be made pretty cheaply these days, or you can always wait for the Black Friday sale at the end of next month and see what sort of deals you can get for a vendor pre-made system then.