Unless you've added a videocard to that system your running onboard video which is more of a bottleneck than that processor. If this is the case, I'd recommend this:
ATI Radeon 4770 512mb video card, and
Gigabyte ep45 UD3R motherboard
Xigmatek s1283 heatsink/fan combo, or OCZ Vendetta 2 Heatsink/fan combo, whichever is cheaper
This motherboard will allow you to overclock that processor to 3ghz and beyond (3.2ghz is average with a decent cooler and safe volts, 1.4v), while that videocard will be approximately 125x faster than your current onboard video. Total cost? A hair above $200 usd. The speed of the 2180 overclocked will be similar to a stock e8400, and you can't really overclock with that board. The videocard will be such a massive improvement I can't really put it into words for you here.
EDIT: I see you have a 9800 gtx which is sufficient. I still recommend the motherboard/heatsink upgrades over the processor upgrade. That 2180 is an overclocking champ but that motherboard is a dud. Note: you could overclock the processor reasonably well with that motherboard and save some money using a "bsel/pin mod" but that's somewhat risky and requires some research and such - I'm thinking you probably aren't interested in that route.
If you have further questions dont be afraid to pm me, I've been a pc hardware enthusiast for as long as I can remember and I love this sort of stuff.