Build your own. It'll be half the price and much better balanced than the usual "OMG 12GB of RAM" and integrated graphics.
Don't buy an All-in-One, the premium is large compared to normal Dells and astronomic compared to building yourself. Micro-ATX boards are already small enough to be unobtrusive.
Can you tell us what the computer will be used for? Which applications, which games, and for the games what settings and resolutions do you use?
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Assuming general office tasks (nothing too intensive), and gaming on High at 1600x1200 or thereabouts:
CPU: Phenom II X3 720 ($120)
Graphics: Geforce GTS 250 ($110)
OR Radeon HD5750 ($145) if you're willing to pay a bit more for lower power consumption and noise
RAM: 4GB DDR3-1333 (~$80)
Motherboard: 785G (DDR3) ($80)
HDD: 1TB (~$80)
Case and >=450W PSU (~$50)
DVD Burner (~$26)
Total: ~$550 for case hardware
High-end machine for video encoding, photo editing, and gaming on 1920x1200 Max settings:
CPU: Phenom II X4 955 ($169)
Graphics: Radeon HD5850 ($259) Note: hard to find at RRP now, so the choice is wait a few weeks or pay a $50 premium now. There is no serious Nvidia competition to this; it outruns the $360+ GTX285 by 20-30%.
RAM: 4GB DDR3-1333 (~$80)
Motherboard: 785G (DDR3) ($80)
HDD: 1TB (~$80)
Case and >=450W PSU (~$50)
DVD Burner (~$26)
Total: ~$750 for case hardware
