Question: Does Windows7 32-bit support more than 4G of RAM? XP and Vista 32-bit versions wouldn't see more than 3.5Gb (or 4Gb if you did some tweaking...at your own risk), you'd have to go to the 64-bit OS to get up above 4Gb.
Memory.....Crucial is awesome, never had a problem with any of their sticks, but I found some PNY the other day at
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more! that was a couple bucks cheaper. PNY, Crucial, or Kingston seem to be the best brands I've ever used. There are cheaper sticks available, but I've had those lock my system up.
No arguments on harddrive choices! If you get a motherboard that doesn't have dual SATA/IDE harddrive support, I found some IDE-to-USB converters on Amazon.com for like $20. I've got both of my old DVD drives (player and a burner) plugged in to my new computer with the crappy DVD burner.
For video, the general thumbrule I've always gone by is if you're playing games, go nVidia (GeForce). If you're doing graphics, go ATI. If you're doing both, I default to nVidia, and have never been sorry.
Motherboard/CPU...definitely get a dual-core or quad-core (you can sometimes snag sales where the quads are same or cheaper than duals), and the whole Intel vs AMD debate is pretty much just personal preference. I've never had a prob with either...my current system is running Intel, my old MAME system is running AMD. And its been some years since I've bought a motherboard, but ASUS was always a top brand. Make sure its got enough slots for what you want to add (PCIe is the latest in video...make sure your card matches your motherboard. I don't think this will be an issue, since everything I've seen coming out recently has had a PCIe x16 slot in it, and two or three more PCI's for sound or whatnot).
I'd also recommend dual optical drives, one player and one burner, unless you spend an obscene amount of time burning CD's/DVD's. Not sure if slapping a Blu-Ray in there is worth the price or not, that comes down to budget and taste, but at least get a DVD burner and player.
Building your system is fun, and you shouldn't have to worry about compatibility issues like we did waaaaaaay back in the day, but I've bought boxed systems off-the-shelf for my last two PC's. The prices are cheaper, even when I had to buy more memory or better vid card. So I wouldn't discount boxed systems just yet. But, as I said, building is more fun!