$300 budget gaming machine while utlizing XCLIO A380 silver case

APPROXIMATE PURCHASE DATE: ~2 weeks BUDGET RANGE: $300ish Cheaper the better

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Gaming, Coding, Movies, Internet usage

PARTS NOT REQUIRED: Case, RAM, Keyboard, Mouse, Monitor, Speakers, OS

PREFERRED WEBSITE(S) FOR PARTS: www.newegg.com or www.tigerdirect.com

PARTS PREFERENCES: Most likely AMD processor and ATI graphics

OVERCLOCKING: Maybe (not for the initial build, but I would like the capability there if it doesn't add to the cost)

SLI OR CROSSFIRE: Maybe (not for the initial build, but I would like the capability there if it doesn't add to the cost)

MONITOR RESOLUTION: 1280x1024 (for now, monitor will be upgraded in the future)

ADDITIONAL COMMENTS:

I purchased a XCLIO A380 Silver Full Tower Computer Case with P5NSLI mobo and 500W PSU from ebay a couple of years ago. I was going to put intel quad core CPU, but the rapid change of tech made the barebone sytem old and made me look into a new build. Basically, I don't want to spend more time working on the old mobo and want to build a AMD gaming system. I read other thread about building $300 budget PC and got some idea on the part that I want to put to this build.

1. CPU & MOBO combo $140 http://www.newegg.com/Product/Comb [...] mbo.237632

AMD Phenom II X2 545 Callisto 3.0GHz Socket AM3 80W Dual-Core Processor Model HDX545WFGIBOX

GIGABYTE GA-MA785GM-US2H AM3/AM2+/AM2 AMD 785G HDMI Micro ATX AMD Motherboard

2. LG 22X DVD±R DVD Burner Black IDE Model GH22NP20 - OEM $25

3. SAPPHIRE 100254HDMI Radeon HD 4650 1GB 128-bit GDDR2 PCI Express - $55 http://www.newegg.com/Product/Prod [...] 6814102843

4. Western Digital Caviar Blue WD3200AAKS 320GB 7200 RPM 16MB Cache SATA 3.0Gb/s 3.5" Internal
http://www.newegg.com/Product/Prod [...] 6822136074 - $50

The estimate price is $278.98 including shipping.

I need your advice whether this direction is worth to throw the old P5NSLI mobo away and build a fresh build with the case.

Thanks!

I will go and check the PSU spec. Meanwhile, would there be a no problem putting the Micro ATX AMD Motherboard to my full case?

The PSU installed in the barebone is Logisys PS575XBK. It says that the total power is 450W and the max output is 575W. Is this a decent one that I can keep or not?