I honestly don't think you are going to build this for $300. BTW you can drop the heatsink and fan. Have you looked at Newegg and Mwave.com.
I'm curious about the case too. I wonder if that Dell case is proprietary.
Can't drop the heat sink and fan, combo DOES NOT come with one. Just looked at Newegg and Mwave. Changed the order a bit:
RAIDMAX SMILODON Extreme Black ATX-612WEBP 1.0mm SECC Steel ATX Mid Tower Foldout MB Computer Case With 500W Power Supply - Retail 59.99
OCZ Platinum Revision-2 2048MB PC6400 DDR2 800MHz Dual Channel Memory (2 x 1024MB) 29.99
Thermaltake TR2-R1 / AMD Socket AM2/939/754 / Aluminum / 92mm Fan / CPU Cooler 19.99
EVGA nForce 730a Motherboard CPU Bundle - AMD Phenom X4 9550 Quad Core Processor 2.20GHz OEM 199.99
Total $310 and the customer is
Bald, the AMD 9550 tests quite a bit better than the E8400 according to Toms hardware in the areas that this system will be used for.