This is just IMHO, but for your budget and what you're asking for, I would recommend looking into a site that builds custom rigs for you like iBuyPower, CyberPowerPC, etc. if you don't / won't / can't build it for yourself for any reason. ;O Just remember, it's ALWAYS cheaper to pick components out yourself and build it by hand -- but I know that's not feasible for everyone!
Of course, the answer is dependant on how much horsepower you'll want for your design work -- if it's an intensive program (or set of programs), you'll want a powerful, multiple-core processor and plenty of RAM, and a solid GPU -- maybe with good VRAM for multi-monitor setups if you're into that or need that for work.
Keep in mind, while you could get a fairly awesome i5/i7 Sandy Bridge rig for that budget, you may not need that much power -- and if you don't need then you could easily build a gaming/creativity rig for around or under $500-$600ish that will not only be effectively cheaper than buying a prebuilt from HP or what have you, but more powerful. Still, future-proofing is never a bad idea!
More bang for your buck, if you will.
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