Well to give you an idea, of what around 2k can get you the rig i built below cost me just around 1800 for everything you see, monitor included. Playing on a 26in widescreen is amazing btw.
Depending on how long you have to go til you can afford to upgrade again should sway your decision between going for dual core or single. In mine I chose to go single for the current gaming market. Since new socket sets and quad core processors are on the horizion the current dual cores will be dropping soon. If you figure that you can make a 3700+ last for around 7 months before you will see any games that will take nessisary advantage of dual core where you will be missing key things in the game dropping 233 now wouldnt be that bad. By that time the FX-60's should even be down around 500 since they are 1k now.
With Windows Vista being delayed you will get a little more out of either the 7900 or x1900 graphics cards that are on the top of the market right now. Personally I would recommend the x1900 because they are designed for heavy shader useage and HDR effects with AA on. However if your an Nvidia fan than the 7900 cards will not disapoint they have a much better price range even though most places are sold out right now you should be able to find one somewhere. If you get 2 GT's in SLI your golden 1 GTX golden 2 GTX's and you hurt my feelings
939 sockets are still on DDR for the most part and the new sockets they are making will be using DDR2 so keep in mind if you upgrade your board for a processor later than you will have to get new RAM as well and CPU just an extra cost but yet it happens, one of the benifits of current intels are that you can use DD2 in the next gen of mobo since it doesnt seem to be going away too soon.
Everything else depends on personal taste, how much you wanna spend on sound, keyboard, mouse and such is kinda hard to justify if your using headphones I wouldnt say get a super XF-I mega edition or something.
Remember though its your rig build it how u want it.