SLI is not cost effective at any price point. The only time you should realistically consider SLI is if you are able and willing to buy top of the line, wherein you don't mind dropping $500+ for a sub 10% increase in frame rate. At the moment, you can nab a 9800GT for <$150, for instance.
Something else to keep in mind is to always get ram in even amount of sticks. DDR is a nice thing, and there is no reason to not go for it. Unfortunately, this generally precludes weird amounts of ram like 3gig. I don't think anybody makes 1.5gig sticks. You would be better off with 2 gigs (2x1gig) or 4 gig (2x2gig, or 4x1gig (64bit systems only)).
Quad core is also more of a future proof move than anything; there is a shift to making more application use parallel processing, and some apps currently do (compilers, renderers, video editors) and some games do (supreme commander, demigod (most likely, since the game is based on the SC engine)). However, be a little wary - while it is a future proofing move, by the time something like quad core is the new standard (as duel core is today) there will be better and cheaper alternatives. Unless you really need those extra cores, you'd be better off with duel core at the moment, in terms of price vs performance.
