Major hardware overhaul advice/help requested

[Aerozord]

ok my computer has been running like crap, and rather then narrow down why, and since I am overdue for upgrades, I decided to just fix every potential issue at once.

What I'm doing
Replacing 160gig harddrive with a newer 320gig
Buying, and swapping out processor/motherboard
Getting atleast 4gigs of RAM

What I'd like suggestions on
What to look for in motherboard and CPU. One of the possible problems with my system is I screwed up when I bought them. As well as any common mistakes when purchasing them. Can already tell finding one with a AGP slot will be the biggest problem. If I have to I could probably sell my current video card, so it wouldn't be too bad getting a new one

I think I got RAM figured out but its been a long time since I got any so if there are any recent changes please share.

With the harddrive, what are some ways I can easily, or quickly, swap files onto the new one?

[Azisien]

Easiest way to swap would be to either buy, or borrow, someone's external hard drive. Most drives are bigger than your drive anyway so it wouldn't take much.

If I were to suggest hardware I would definitely need budgeting specifics.

[Aerozord]

300 total, I'd be willing to go 400 if I have to get a new video card, but only if its atleast one par with my HD radeon 3850

[edit] if motherboard says DDR 2 standard, does that mean I cant install DDR 3?