From what I can tell, it is going to be tough to get a machine for $550 w/t an actual graphics card, if that price needs to include a monitor as well. Especially at a retail store.
I was able to build this Dell (small business dept) for about $420
Inspiron 546 Minitower
SYSTEM COLOR Piano Black
PROCESSOR AMD Athlon™ II X2 215 (2.7GHz, 1MB)
OPERATING SYSTEM Genuine Windows® 7 Home Premium, 64bit, English
OFFICE SOFTWARE Microsoft® Works 9 (Does not include Microsoft Word)
SECURITY SOFTWARE No Security
WARRANTY AND SERVICE 1 Yr Ltd Hardware Warranty, InHome Service
MEMORY 3GB Dual Channel DDR2 SDRAM at 800MHz- 3 DIMMs
HARD DRIVE 320GB Serial ATA Hard Drive (7200RPM)
OPTICAL DRIVE 16X DVD+/-RW Drive
VIDEO CARD ATI Radeon HD 3450 256MB
SOUND Integrated 5.1 Channel Audio
SPEAKERS Dell AX210 1.2 Watt 2.0 Stereo Speakers
KEYBOARD Dell USB Entry Keyboard
MOUSE Dell USB Optical Mouse
None of the monitor bundles were worth it. So you'd want to pick one up separately. You should be able to get a decent 20" for the remaining $130.
Most of the other systems I tried to configure w/t a graphics card, wound up around $580 or so with a monitor. The small business Vostro line is one of your best bets.
With Dell I tend to favor their Small Business dept for computer purchases. Their tech support for those computers is MUCH better than their consumer dept. You don't need to actually be a business to buy from them.
If you do go with a retail store, I would try to avoid anything slim line or narrow. You definitely want some sort of a mini tower case.
