Upgrading a prebuilt computer

The fx-4100 is crap and this is pretty overpriced. even without the cost of a gtx 550ti you could build a better system and not need to spend $100 on a GPU. What is your budget?

~800-850$. that's including the monitor and windows 7

PCPartPicker part list / Price breakdown by merchant / Benchmarks

CPU: Intel Core i3-3220 3.3GHz Dual-Core Processor ($109.99 @ NCIX US)
Motherboard: MSI B75MA-P45 Micro ATX LGA1155 Motherboard ($54.99 @ Newegg)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory ($45.99 @ Newegg)
Storage: Western Digital Caviar Blue 500GB 3.5" 7200RPM Internal Hard Drive ($57.99 @ NCIX US)
Video Card: HIS Radeon HD 7850 1GB Video Card ($176.97 @ Newegg)
Case: Cooler Master Elite 371 ATX Mid Tower Case ($37.20 @ NCIX US)
Power Supply: Corsair Builder 500W 80 PLUS Bronze Certified ATX12V Power Supply ($51.98 @ Newegg)
Optical Drive: Lite-On iHAS124-04 DVD/CD Writer ($16.99 @ Newegg)
Monitor: Acer G236HLBbd 60Hz 23.0" Monitor ($130.63 @ Amazon)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ Outlet PC)
Total: $772.71
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-01-07 04:04 EST-0500)

This will be much better than the prebuilt and a 550ti.

I left some space in the budget in case you want to go to an i5 or a gtx 660/ HD7870 or want a different monitor.