Im goin to post my opinion to you because, well... PCs and tweaking them is my thing... Just like tuning car is for other men.
Right now, if you were to upgrade your CPU only, just dont. What you have there is fine, however if your looking for a platform upgrade, Intel is the way to go right now.
Ive seem posts here claiming AMD is for gaming and Intel isnt... Oh posh, posh, that is balloney...
Current Intel C2D architecture make them superior in IPC (Intruction per clock cycle than AMDs offering). They also run faster and cooler... Im curently running a B2 revision E6420 overclocked with ease to 3.2Ghz... Im planning on getting a Xeon E3110 later so I can setup a second rig and sell it to a friend...
The only advantage AMD has right now, is in a complete platform segement, then own GPU/Motherboard and CPU combo segments, but lets be real here... Your a gamer and want the best. Right now, the only way AMD is cheaper than Intel is in the Chipset area... You can easely find half decent AM2 board for 45bucks, not so on the Intel side... CPU wise, I can find you cheap E2160 that clocks to 3.0Ghz and blow anything AMD has in its price segment of 70bucks.
Now that I put that aside, as a comsumer its your duty to get the best bang for buck, right now Intel/Nvidia have that... Theres no point in being "loyal" to a compagny, they really could care less about you... The point here is that you actaully get your moneys worth out of something no matter whos leading in what industry.
Right now a "proper" setup would consist of a Dual Core 45nm part coupled with some cheap DDR2.
Motherboard choices are as follows:
Mid-high end:
Abit IP35Pro P35 Chipset
Asus P5K-E P35 Chipset
High end:
Asus P5K Premium Wifi/Ap P35 Chipset
Asus P5E X38 Chipset
Asus Commando P965 Chipset
Ultra high end:
Asus Striker II Extreme or NSE 790i Chipset
Asus Rampage Formula X48 Chipset (highly binned new rev. X38)
CPU:
Your a gamer and honestly unless your dual logging, doing serious multitasking or 3D rendering/encoding, you dont need a quad core;
However if you do, I still have options for you:
Low-Mid end:
E2160
E2180
E4500
Mid-High end:
E6750
E6850
E8400 (Highly recommended 45nm part)
X3110 (Xeon, basicaly the same as E8400, save for a bit fo microcode)
Q6600
High-Extreme end:
Q9450 (Highly recommend 45nm part)
X3350 (Xeon same as Q9450)
E8500
Q9550
(notice no "extreme" series processor, you dont need them at all... unless you like to play 3Dmark06 and SuperPi under phase or LN2)
For RAM, Id take a look at the 2x2Gig kit from Crucial, the Ballistix PC2 6400 and the Corsair Dominator PC2 8500 kit that both have the saem type of Powerchip ICs, make them one of the BEST overclockable 2x2Gig RAM kits around...
Ah, noticed you want SLI... I hope you got a big LCD to make use of that, In that case... Go for the Nvidia 790i platform, its superior in all aspects to any other Intel based Nvidia Chipsets... Youll need some DDR3 tough, but that not too bad... There some "cheaper" but awsome Patriot 2x1Gig DDR3 1600Mhz RAM kits out there selling for like 300bucks, all of them using the highly overclockable Micron D9GTR ICs, which on the right mobo can hit over 2000Mhz "effective" (hinting Nvidia MCHs here)
For SLI, the best scaling and cheapest uber performing setup is 2x 9600GTs
About 300bucks and just pure awsoness for the price
Ok, that sums it up for now.