Well... I would say 512mb is considered below the min these days. For editing and especially with running vista, I highly recommend a mininum of 1GB of ram.
I'm not sure how much or what kind of video editing you intend on doing. If you are importing digital video off a camcorder and editing with Adobe Premier for example, then your requirements just jumped up a bit. A firewire connection on the mothboard can be used to import video, you will need min 1gb ram, dual-core processor, and a fast storage system with lot's fo storage. Video takes up a LOT of room on a hard drive while editing. I suggest a large capacity drive in a raid setup. Increases speed, which will be needed if editing large videos.
As for the video card, you could spend your entire budget on it... I wouldn't say it's the most important thing, as it doesn't affect video editing as much as playing games.
Here's something I quickly put together
Processor :AMD Athlon 64 X2 4600+ w/ 2x512K Cache (Socket AM2) 1 $169.95
$169.95
Motherboards :Asus M2N-E SLI w/ DualDDR2 800, 7.1 Audio, Gigabit Lan, 1394, PCI-E x16 SLI 1 $124.95
$124.95
Memory :Kingston 1GB PC2-6400 DDR2 SDRAM 1 $169.95
$169.95
Power Supplies :Antec TruePower Trio 550W Power Supply w/ Triple +12V 1 $109.95
$109.95
Hard Drives 1 :Seagate 500GB Barracuda 7200.10 SATA II w/ NCQ, 16MB Cache 1 $209.95
$209.95
Hard Drives 2 :Seagate 500GB Barracuda 7200.10 SATA II w/ NCQ, 16MB Cache 1 $209.95
$209.95
Video Cards :eVGA e-GeForce 7600 GS 256MB PCI-E w/ Fan, DVI, HDTV-Out 1 $129.95
$129.95
Video Capture Cards :ATI TV Wonder 650 HDTV / Analog TV Tuner, PCI 1 $139.95
$139.95
Sound Cards :Onboard 8-Channel 7.1 Audio 1 $0.00
$0.00
Ethernet Cards :Onboard 100/1000 Gigabit Lan 1 $0.00
$0.00
Operating System :Microsoft Windows Vista Home Premium x86 (32-bit) DVD, OEM, 1-Pack 1 $139.95
$139.95
Total: $1488