I also should add my main games are CS:S, WOW, Crysis, COD4 and GTA4. Average results:
Counter strike source maxed: 250 fps
WOW maxed out: 60 fps
Crysis all on high: 30 fps
COD4 maxed: 90 fps
GTA4 mid/high: 32 fps
you could grab another 9800gtx for SLI or just buy a next gen card like the gt295. Maybe get an aftermarket heatsink to overclock your cpu to around 4ghz. You could also set up a Raid 0 array for your games to load faster. Other than that your rig looks good.
Oh, just noticed you said your headset is crap. You could get a nice set like the Plantronics Gamecom 777 with the built in sound card. I have the gamecom 377 and its awesome (I already had a sound card). Then spend the rest on some new games!
If that was my rig, and I had an extra $800 to blow, here are some of things I would look at buying:
New golf clubs
New fishing rods and tackle
Bob's Outdoors has a couple of really nice shotguns on sale right now..
New Brakes for the truck.
New lawn mower.
New kicker motor for the boat.
New storage building for the back yard.
But probably the best thing to do with the $800, put it aside for a rainy day. There are probably going to be a lot them over the next several months.
Your computer is better than 99% of the people out there have.
Why you want to dump another $800 into it is pretty foolish, IMHO.
A few things to add: my monitor is a new purchase (i upgraded from 19") so im not planning on getting a new one
also, the PSU did not come with the case, its a 700W that i purchased seperately, i just forget the name.
As for SLI, those of you that have the 9800GTX+ know that its MASSIVE! with my possible sound card and wireless adapter for when my dad unplugs me in the middle of a scrim, theres not much space left. I think that to run SLI, i would neen a different motherboard (bigger)
If you are really set on spending all that money then just get the fastest core2quad your mobo will support and put the rest towards upgrading your graphics. You can either ditch the wireless pci card for a usb adapter so you can fit SLI or just pick up either a gtx295 or HD 4870x2.