Well, I'm not a guru by any means, but I did just put together my own computer from parts and it was worth it. Not only did it save me over $100 or so dollars, but I learned a bit so if I ever need to do it again it will be much easier. As far as I know the x86 version of windows has less hardware conflicts and supports up to 3.25GB RAM (correct?), so if you really need that extra 750mb it's your call. Personally I get away fine with 2, but I will be putting more in it just because I do hit that when playing games. I got the 8800GT and it looks like it's the best bang for your buck right now. With just one it runs Oblivion at 1680x1050 with all settings maxed (using HDR lighting, so no AA) at 50-60fps average. So unless you're into extreme semper games (yeah, I said it) I don't really see a need to run anything in SLi.

Getting vista JUST to be able to use all 4gb of your ram is a horrific idea. Ram doesn't have that much of an impact on your system after you get enough of it. Unless you run a server, or do CAD or video editing, you will not notice a difference over 3gb. Actually, considering how much more ram vista takes up, you could even see LESS of that 4gb.

Also a lot of games only work in 32 bit mode and you need to utilise 64 bit processing to break the 3.5gig RAM mark.

Not to mention, you would have to get 64 bit versions of ALL your applications, drivers, and games. And if they aren't available in 64 bit, you can't run them.

yup

Quick short answers to your questions: don't SLI, don't crossfire, get a ATI 4850 because they are cheaper better performers compared to Nvidia's offering. The 4870 is even better but that's if you really want high end shit. I bought my computer through CyberPower a LONG time ago and while I had some problems I pin most of them on the fact that I was inexperienced. I didn't get an OS on it, didn't have a floppy drive (which was a bit of a problem back in 2004), and I fried my video card because I didn't handle it right. If you are really concerned about CyperPower, look them up in the BBB.

Get Vista as it's more secure and has more features. If you prefer the uglier XP I suppose it's cheaper and games tend to perform better on XP, but you'll be missing out on some of the nice things Vista brings to the table. XP Pro can detect up to 4gb RAM but you must factor in that your system counts any RAM on your videocard towards your max RAM. For instance, on a 32-bit system: if you have a 1GB video card and 4GB of RAM your system will use only 3GB of RAM and the 1 GB on your video card. 64-bit systems don't have this problem.

Sorry I typed that all VERY quick just ask if you want elaboration on anything.

http://en.wikipedia.org/wiki/Features_new_to_Vista

I won't touch on all the features, but some really are invaluable, like the ability to hit the window key and type in a program's name you want to launch (or even part of the name).

For instance, if I want to open up Trillian, I can hit window, type in "tril", hit enter, and voila, it's open.
Windows explorer got a nice overhaul too, with new things like thumbnails for Word documents, scalable icons for pictures, etc.

EDIT: Also more about CyberPower

XP Pro can detect up to 4gb RAM but you must factor in that your system counts any RAM on your videocard towards your max RAM. For instance, on a 32-bit system: if you have a 1GB video card and 4GB of RAM your system will use only 3GB of RAM and the 1 GB on your video card.

nope, only if you're using a card with shared memory