OK, that power supply is more than enough-I run off a 450W and do fine.
SLI really isn't just that useful-it is flaky at times with game compatibility and has always been a backburner driver priority to nVidia. It also generally is better to sell your old card and buy a new card instead of adding in another, older card in terms of speed. Your motherboard may have SLI on it, but that's just because of escalation in the motherboard market and good chipset support.
Anyway, what I would suggest is this:
RAM-$80- 2GB (2x1GB) OCZ DDR2-800 from Fry's -
http://shop3.outpost.com/product/50083152GB of RAM is essential for a modern PC. 4GB wouldn't also be a terrible idea given how cheap the stuff is, the problem is to use it all you have to go to a 64-bit OS, which sucks ass. So 2GB it is!
Motherboard - $130 - Gigabyte GA-P35-DS3R LGA 775 Intel P35 - Either from Newegg, ZipZoomFly, or ClubIT, all whom I recommend.
This motherboard is based off the latest chipset and is future proof for the upcoming 45nm Intel CPUs so that a future Fry's trip in a couple of years could bag you a sweet, easy upgrade. Also, it has no shitty onboard Realtek-the onboard audio is pretty decent. No SLI, but again that is so something you shouldn't consider given your suggestion to keep the heat down. I run the 965 chipset equivalent board in my gaming PC and it is excellent.
Videocard - I'm going to give two choices here.
Choice A ) 8800GTS 320M - This is if you are running a 1280x1024 LCD or so, and aren't planning on upgrading it in the next 18 months. If so, this card is a freaking beast for the cost ($270 currently). They actually don't run all that hot unless you compare it to a super-cool (and somewhat underpowered) 7900GS or what not.
Choice B) ATi X1950Pro 256MB - This is the budget choice, but it is still really fast for DX9 games and will serve you very well for a good 18months with above-average (but not maxed) settings and goodf framerates. Runs pretty cool as well. $130 or so.
I'd say go with the X1950, it's a good card and you're not the kind of GAF distinguished mentally-challenged fellow that refuses to play PC games if they can't crank everything to max.
For heatsinks-I would go with a Zalman, as others have suggested, or the overclocker's (and my current one) budget favorite, the Arctic Cooling Freezer 7 Pro.
Regarding the 256/512 situation-The thing is that you might gain performance by going up to 512MB from 256-and you most certainly do with textures cranked-you wind up moving up almost an entire class of GPUs for the premium. A 320MB 8800GTS costs about as much as a X1950Pro 512MB, but the performance of the 8800GTS is going to sodomize the X1950 Pro in the majority of circumstances.
If you want to run with a ton of RAM, go for a 8800GTS 640MB/8800GTX and be done with it.