If you really have up to 5k to spend, you definitely want to get an AMD dual core processor. (The 64x2 product line.)
I personally think that the 4200 there is the sweet spot for price/performance, but it sounds like you have more money to burn than I do, so you might even want to jump up a spot or two.
I'd start with that, find a nice MOBO that meets your needs, find 1 or 2 gigs of RAM (2 is better, but 1 is still great - and yuou can always drop more in later) and get a video card (the 7800 seems to be popular around here)... and then you've got the guts of your new rig. Just add case, PSU, monitor, speakers and hard drives.