Moby And RAM Reccomendtions

[Paft]

Story in short:
So, something isn't right with this comp which hasn't been from the start. Some 1080p videos can be out of sync and games play like crap (TFC, FF and Q3 - Only games I play). Also an external HDD disconnects during file transfer on this machine only.

When I first put it together I was on a P4 CPU for a little while. I have also been 3 different HDDs since. I have also been on XP, tried Vista and now on 7. And it still runs exactly the same.

I am a little out of the game so I'd like your suggestions.I am looking to spend about £140 including VAT + Delivery


Mobys:
Store 1
Store 2

RAM:
Store 1
Store 2


Current Specs:
Cooler Master Centurion 5 Silver
Arctic Cooling Freezer 7 PRO (Socket LGA775)

Intel Core 2 Duo E4600 2.40GHz (Retail 775)
Asrock CONROEXFIRE eSATA2 775 Motherboard
OCZ 1GB PC2-5400 Gold GX Edition (2x512MB)
Hiper 580W Modular PSU BLACK
nVidia GeForce 7600GT 256MB PCI-E