SLI isn't worth it because two cards cost more than a single card of a higher generation or grade, which beats two older cards in SLI mode anyway, let alone the fact that while you get more joint bitrate (sort of like Dual Channel for RAM, no?) and more RAM over two cards, there are just those things the card won't do, e.g. the hardware support of this or that. If you like experimenting, I believe you'd be better off with custom cooling and big OC (maybe volt mod or something) on your current card or even a new card. Still, I doubt you really need an 8800. Besides, 8600 is barging in. I believe 8300 has been presented as well. 8800 cards will get quite a lot cheaper quite soon, as I predict. Especially the moment ATI comes up with DX10 cards. Perhaps get some cooling etc for your 7900 that will work on an 8800? You then just sell the card and get the 8800 when it's cheaper and use the cooling from your previous card, and overclock (as there will likely be menaces of 9900 by that time, eh).
As of now, I believe you need RAM. The fastest your mobo supports and the processor can handle. Two gigs if you want, but not more. You can sell your current chips and buy 2 * 1GB, preferably a matching kit, the fastest you can get - your mobo may support this or that frequency as the maximum, but within one frequency, you have chips with different latencies, e.g. CL 2.5 or CL 3 sticks within the DDR400 range. I don't know to what extent fast RAM and CPU-RAM exchange rate affects a gaming rig's performance, but going for 1:1 rate at the lowest timings possible is always a good idea (and a sensible margin to allow for higher FSB from overclocking is a good idea as well) - it's just not worth overpaying.
Another suggestion would be to look at ebay and used component markets and see what you can upgrade cheaply, i.e. pay not much more for what you buy than you will get for what you sell. You can always buy a new component first and then wait for a good price on your old one (i.e. wait long enough for the bids to rise well, don't hurry and sell for any cash, but don't wait until the price of that component class falls, of course).
And what kind of soundcard do you have? Maybe you'd like your aural experience to match your visual one and at the same time take some load away from the CPU? I've recently got the "revelation" that pumping up the graphical side of one's rig while neglecting the sound isn't really such a great idea, nor does it allow full enjoyment. At this point, I'd rather get an X-Fi Gamer for myself than a new CPU or a new graphics card, I think.