[New System Build] New Cheap Gaming Computer

I am building a new pc with money from selling my old Wii and Xbox games.
I already have these parts from my old computer:

585w Power- http://www.newegg.com/Product/Prod [...] 6817339012
Sapphire 3850- http://www.newegg.com/Product/Prod [...] 6814102794

And here is what I am buying and trying and stay under $300:

If I buy a case here is what I will get:
Case- http://www.newegg.com/Product/Prod [...] 6811156223
MOBO- http://www.newegg.com/Product/Prod [...] 6813138130
CPU- http://www.newegg.com/Product/Prod [...] 6819103649
RAM- http://www.newegg.com/Product/Prod [...] 6820227199


If I do not buy a new case I have to go Micro ATX and get just these:
MOBO- http://www.newegg.com/Product/Prod [...] 6813157162
CPU- http://www.newegg.com/Product/Prod [...] 6819103649
RAM- http://www.newegg.com/Product/Prod [...] 6820227199

How will this work?
I will be doing some gaming that I dont want any lag in, problably not too much Crysis , and lots of video editing and web surfing.
Should I step down to an older Phenom X4 to save money?
Will the Phenom II X3 unlock X4 thingy work with this board (790GX North Bridge)
Could I do some type of Crossfire with the 3300 onboard and the 3850 I have?

Thanks for all the help and I hope this is not too confusing.

The HEC (Orion) power supplies have a pretty poor reputation, normally i'd say switch it out, but since you're going for a tight budget system, i'd keep it. Just be cautious of any problems you might have (BSODs, black screens or soft lock ups) they could be power related.

Look at the 2 combo deals available with the 720.
An Asus AM3 (uses DDR3) 790GX motherboard $195 combo deal
http://www.newegg.com/Product/Comb [...] mbo.205337
A Biostar AM+ (uses DDR2) 790GX $194
http://www.newegg.com/Product/Comb [...] mbo.200066

RAM for the Asus AM3 $65
http://www.newegg.com/Product/Prod [...] 6820231200

RAM for the Biostar AM2+ $55 (free shipping)
http://www.newegg.com/Product/Prod [...] 6820231166

1. All your parts are compatible.
2. Don't step down to an older Phenom, if you have to, drop to the PhenomII x2 550
3. Biostar should still have a BIOS version that MAY be capable of unlocking the 4th core of the 720, but don't bet on it.
4. You will not be able to use the HD3850 for hybrid crossfire.
5. The HD3850 is an aging and lower middle graphic card, don't expect too much out of it.

Hope this helps.

bump, I have to keep the GPU and PSU for now because they are both working fine for me. (My psu had many good reviews.)