I will be upgrading my PC in May. It's a gaming PC. I have a couple selections. It can only be one.
My Choices are:
Phenom Processor
Antec 900
24" LCD Monitor
Add an 8800gt and SLI it
(OTHER RECOMMENDATION?)

Due to the fact that many games do not take advantage of the quad core processor, I would not go with the Phenom. My personal recommendation would be to get another 8800gt and SLI.

What computer case do you have? 1 8800 GT is plenty for now, and if I were you I would wait until the next wave of video cards comes out before upgrading video cards or processors, especially since there is no phenom worth purchasing at the moment. I say this as an AMD fan that has been forced to purchase an Intel chip. If your case is less than spectacular then upgrading to an Antec 900 would help. Also upgrading your power supply to a PC P&C Silencer 750w or equivalent would future-proof you for a long while. Lastly depending on your current monitor if it does not go up to atleast 1680 x 1050 resolution then you might consider upgrading. The rest is on unstable ground, so I truthfully believe a wait and see approach would be best. If you upgrade your post and tell us what your working with at the moment I can then give you better advice.

Hope this helps and good luck.

Did no-one look at the sig of OP's rig?

Gaming wise Phenom is a waste, with its issues and lower performance to that of Intel your X2 will suit you fine for a while yet...

Adding a second GPU will not help your cause if you have a small screen ie 19" as SLI only proves usefull on high res's. So i believe that to be a waste also.

Case depends on what you have now ... but the Antec900 will do you for a while.

and 24" depends on what you currently have if its small low res then this might be the goer... i love mine Dell 2407HC

I have a CoolerMaster Centurion 5 case. My monitor has a max res of 1440x900

In that case i would ask why you want to upgrade? Save the money for a new build down the track... hell take your gf out to dinner or something