Build your own system. Use a buyer's guide (Anand's is my personal favourite, but there are lots of good ones out there) and follow it as a guide, change what you like. Save the rest for upgrades if you have any left.
Personnally I'd get something on the order of a Thunderbird 750 with Asus A7V or ABit KT7, 256MB of RAM, a pair of 30 or 45Gig IBM 75GXPs in RAID0, Pioneer 16x DVD (w/ hardware decoder, but that is 100% personal preference, a 700 would have enough juice for software playback), Plextor 12/10/32A, a GeForce2 and an SBLive (any flavour). This won't be $5000 but it will be more than enough power and won't waste any cash, and put into a really nice monitor (21" sony flat screen) so you won't have to upgrade that for a while. And what you have left save for upgrades. Oh yeah, and Video Logic Sirocco Crossfire speakers.
Edit: There is nothing wrong with the P3 as a choice either, but the thunderbird has better price/performance, and even if I had $5000 sitting in my pocket, I still wouldn't want to waste money. If I could get the same performance for cheaper.
If you just want a massive system or can't save it, get something like a P3 933 with an i815, 512MB of RAM, GeForce 2, a couple of SCSI hard drives in RAID, an SBLive! Platinum, Video Logic Desktop Theatre DTS speakers, a 21" or 24" monitor, a SCSI CDRW and DVD...
Either way, build your own, don't buy a prebuilt system from anyone.