Archlight is on the right track, but I'd offer a couple tweaks:
Since you mentioned you'll be shopping at Microcenter you can grab an i5-3570K and an AsRock Pro4-m
for $260 total. Great combo, I've been perfectly happy with mine (see signature). Or the i7-3770K is on sale for $230 right now. That brings your total with the Pro4-M to $300. The i7 won't really get you any better framerates though, I'd classify it as more of a luxury purchase for your use.
A 2x4GB = 8GB kit of DDR3 1600MHz RAM with a voltage of 1.5V or lower typically go on sale for around $40 at newegg. Wait for sales and just make sure reviews are decent before buying. The specific brand or kit doesn't matter much at all. Here's a low-voltage kit from Mushkin
that would do just fine.
GTX 660Ti is a great performer and delivers smoother framerates than the 7950
so that's a great choice. It beats the pants off your existing 5770 which is roughly equivalent to a 7750
Why not grab a 2+ TB hdd. 2TB on up pretty much have the same cost/GB so it's just a matter of how much space you need for your movies and music. If you've got a 240GB SSD, you probably wont have to store any programs on your mechanical hdd so a 2TB 5400rpm drive like this
is a great option.
Case is really up to you, we can help make suggestions based on whether you prefer quiet cases, more airflow, ATX, mATX, etc.
Get the Hyper 212 EVO
since it has an improved heatpipe layout and fan. On sale for $25 after MIR
at newegg right now.
Main: i5-3570K, ASRock Z77 Pro4-M, Asus GTX660 TOP, 120 GB Vertex 3 Max IOPS, 2 TB Samsung EcoGreen F4, 8GB 1600MHz G.Skill @1.25V, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Samsung SSHD, 8GB 1866MHz G.Skill, Hand-Built Wood Case