I'm only 20 now, but I've been editing for about 5 years now (anyone ever hear of Star Wars: Macbeth?) and I've had varied luck with a number of different systems. Right now I am running a single AMD 1.4, 512 MB Crucial, 2x 60GB 60GXP RAID-0 Array, Pinnacle DV500+ capture card, and I don't have any complaints. The first machine I edited on was a PI 166 or something... with a VE500 capture card that produced pretty awful compression quality. I don't really remember the other system specs. Between those two, I had a PIII 500, 256 RAM, 2x 30.0 GB HDs (non-raided), Pinnacle DC30 Pro. I've used Ulead MediaStudio (back in the day), Premiere 5.0, Premiere 6.0, and Final Cut Pro to edit. I really have been happy with the Pinnacle cards, but I've heard that a lot of people like Matrox cards too, particularly the RT2000. I do professional work on my machine, and while it may fall a little bit short of broadcast quality, it's more than good enough for normal professional work.
If you don't care too much about the rest of your system, your should be able to put together something decent based on a Pinnacle cap card for under $2000. The DV500+ runs around $600? I think. The DV500 includes sound capture, so you don't need to worry about a really expensive sound card. With regard to dual processors... dual processors are not as important in video editing as they are in somthing like image rendering. Video editing tends to put more of the workload on the capture card/compressor, rather than on the processor. I could be a little bit off on that, but I'm pretty certain. I'd agree codehack above, avoid the AIW cards. The quality just isn't anywhere near as good. Then again, it depends on what level of work you are going to be doing on this machine.