Need help choosing some parts for upgrades

I built a computer several months ago while on a budget, but it's starting to fall behind in games and is running insanely hot, so I figure it's time to upgrade some parts.

What I have (And want to keep):

PSU- 600w Rocketfish Gaming PSU
CPU- AMD AM3 Athlon II x4 630 @ 2.8GHZ
RAM- 6GB (3 x2gb) @ 1066MHZ in single channel (May sell for G.Skill 1600mhz+ ram)
HDD- 1TB WD Caviar Green w/64mb Cachè
GPU- Nvidia GTX 460 1GB
O.S.- Windows 7 Home Premium 64-bit
Case- HP Micro-ATX (I need to keep this case because of it's size)


So, here's what I picked out for the upgrades. Tell me what you think.

Motherboard- http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Primary HDD- http://www.newegg.com/Product/Product.aspx?Item=N82E168...

CPU Heatsink- I have no idea what will fit my M-ATX case. (Must be air cooling & AM3 compatible)
Dimensions:16.5x7.0x15.25

Dedicated Physx- I was looking at either a GT430 or a GT240, no sure what brand though. Maybe EVGA, XFX, or Gigabyte.


All help is highly appreciated; thanks in advance!

Happy Thanksgiving!