Get yourself a quad and big hard drive - stick with 4GB ram. You should be able to find one like that on classifieds type sites or ebay, and/or refurbished from stores like Best Buy if you’re in the US (and/or Futureshop in Canada).
I got a quad core on Kijiji just over a year ago with a 22" screen for $400, as the guy was moving and had a laptop. So deals are possible.
For Blender, pretty much any video card will work - I’ve used ati and nvidia without problems. Depends on your workflow and how ubuntu and video drivers are going these days (It’s been a couple years since I ran ubuntu)
If you want new, and are willing to save a few more bucks, and are OK with cheaper (but still good) AMD - Go with this. This is all from newegg.com
-AMD Phenom II X4 965 Black Edition 3.4GHz Socket AM3 125W Quad-Core Processor Item #:N82E16819103727
$159.99
-COOLER MASTER Elite RC-310-OWR460 Black Computer Case With Side Panel Window Item #:N82E16811119218 (includes 460W power supply, which is more than enough for the system even with 4 sticks ram, extra sound card, overclocked cpu to 4GHz, and two 120mm fans).
$69.99
-LG DVD±RW SuperMulti Drive Black SATA Model GH22NS50B Item #:N82E16827136216
$15.99
-Seagate Barracuda 7200.12 500GB 3.5" SATA 3.0Gb/s Internal Hard Drive -Bare Drive Item #:N82E16822148395 (note - FAST performing hard drive)
$49.99
-ASUS GeForce GTS 450 (Fermi) ENGTS450 DirectCU TOP/DI/1GD5 Video Card Item #:N82E16814121394
$15.00 Mail-in Rebate Card
$139.99
-G.SKILL 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel Kit Desktop Memory Item #:N82E16820231166
$84.99
-ASRock K10N78D ATX AMD Motherboard Item #:N82E16813157210
$49.99
Grand Total:
$570.93 (minus the 15 rebate)
note - all components were selected based on both price and rating/reviews.