[Build Ready] Cheap, upgradable, gaming pc for GW2 and Dota2

I have a laptop with decent gaming capability but the graphics card doesn't allow me to play half of my library on steam. I am mostly interested in just playing Guild Wars 2 and Dota 2.

I have never built a pc before. I don't know if there are issues with compatability between parts or if I'll suffer in one area because I tried to skimp because of price or something. I just want to build a decently powerful gaming machine for as cheap as possible that I can upgrade in the future if I want to (unlike my laptop). It doesn't have to run everything at max settings.

PCPartPicker part list / Price breakdown by merchant / Benchmarks

Type	Item	Price
CPU	Intel Pentium G630 2.7GHz Dual-Core Processor	$57.98 @ NCIX US
Motherboard	ASRock Z75 Pro3 ATX LGA1155 Motherboard	$84.99 @ Newegg
Memory	A-Data Gaming Series 4GB (2 x 2GB) DDR3-1600 Memory	$29.99 @ Newegg
Storage	Samsung Spinpoint F3 1TB 3.5" 7200RPM Internal Hard Drive	$69.99 @ Newegg
Video Card	Gigabyte Radeon HD 6450 1GB Video Card	$46.98 @ NCIX US
Case	Rosewill R218-P-BK ATX Mid Tower Case	$24.99 @ Newegg
Power Supply	Apevia 500W ATX Power Supply	$21.98 @ Newegg
Optical Drive	Lite-On iHAS124-04 DVD/CD Writer	$14.98 @ Outlet PC
	Total
Prices include shipping and discounts when available.	$351.88
Generated by PCPartPicker 2012-09-03 15:57 EDT-0400

EDIT: Sorry I didn't put a price range. I was thinking 300-500. I want to build it cheap, but I don't want to suffer because I was a penny pincher. From what I gather the graphics card is important but I don't know what to pick.