Heya,
In that case, this is much easier. I'd stick with AMD for this one, since you're on a budget, you'll get more computer for the dollar in this price range. For your needs, I'd shoot for 8Gb of memory and a Quadcore with L3 cache. You can get very decent onboard graphics in today's AMD chipsets, so if you have to wait to get a dedicated card, that's fine.
So you can add/take away to get an idea from the following for what you can get in the $500~$600 price range.
CPU: AMD x4 Propus 620 Quadcore
$99
HSF: Scythe Mugen 2
$35
Mobo: Gigabyte GA-MA785GT-UD3H
$95
RAM: Corsair 4Gb DDR3 1600
$95
PSU: Raidmax 630W Modular
$65
DVDR: Sony Optiarc DVDR
$30
HDD1: Samsung F3 Spinpoint 500Gb
$55
Case: Antec 300 Illusion
$55
Total: $530 ($500 if you drop the DVDR)
So you still have wriggle room in the budget. You can use that to double your memory and go 8Gb (suggested if you plan on doing heavy design work). Or you can use it to get a higher Quad, like the
x4 955 Black $165.
Note that I included the Mugen 2 HSF aftermarket cooler so that you can overclock the snot out of the CPU. I have the 955 and the Mugen2 myself, and can attest that it hits 3.8Ghz with ease.
Later on, you can add an HD5770 for gaming if you feel the need.
For design work, you're going to likely want 3 HDD's. One for your OS. One for your scratch disk. One for long term, finished projects. And an external to transport them with (and for backups). So factor that into your budget as you get into it.
If you already have the DVD drive, you can save $30 off the total cost above and certainly double up to 8Gb of RAM, or go to the 955 Black CPU.
Very best,