Graphic design is quite a wide field. Be more specific about the software. If I were to go for a basic fast multitasking design I'd get:
AMD Phenom 2 (955 x4 processor)
4 GB ram DDR3 @ 1600 mhz in dual channel (frequency and dual channel are important)
~ 100-130$ nVidia graphics with CUDA (I'd say the GT 630 for desktop with 1GB GDDR5 on 128 bit, again, GDDR5 and 128 bit specs are very important) you're gonna need graphic acceleration for basic designing tasks.
get a basic power sorce 450W and two extra coolers, keep the case closed. Have one of the coolers to bring air inside beneath the power source and the other one taking air out. Effective airflow can lower temperature quite a lot (15-30 C, that's huge). and an intel 330 series SSD(get the 90 GB one, should be fine)
But if you're in for the more though things, you're gonna have to go for an intel ivy core i7 quad core processor with hyperthreading, at least a 460 GTX graphic card with 1 GB GDDR5 and 256 bit memory interface. 8 GB RAM @ 2066 mhz (no need for more than 8 GB, seriously) and get at least a 520 series SSD, minimum 120 GB one but I'd recommend a 256 GB one, better case, liquid cooling (not water, liquid)
I currently need portability, so I got a laptop for desining. I use photoshop, adobe premier, heavy rendering, 3DS max, Vray, etc. I need real power in this, so I have:
i7 3610QM 2.3 gHZ turbo boosted at 2.8 Ghz (quad core, hyperthreading to 8 processing threads)
8 GB at 1600 mhz in dual channel memory
GT 650M graphics CUDA card.
120 GB Intel 520 Series Solid state drive.
1920x1080 anti-glare display 17.3 inch.
But honestly, if it weren't for the portability need, I'd totally get a desktop. For example, the phenom 2 takes in 125W power, while my laptop processor is designed to take 45w. everything is laptop designed and optomized, I can't put too much power in it because it's a laptop and it cannot even properly cool down.
For the money I paid for this laptop, I could've gotten 4x times the power in a desktop.
Never buy macs on a budget, I love them too, but If I can't get my investment back in 2-3 months, then it's simply not worth it. In terms of laptops? It's worth it. In terms of desktop, you can get the tower, that's real power but NEVER buy iMacs. They're desktop-ish laptops, basically. Not using much power, not very bright for though tasks.
For example, the latest series as 650M, a LAPTOP graphics card, the processor isn't that bright either, neither is the cooling. iMacs are home-use "Desktops". Never buy them for work.
These are my two cents, hope I helped
