40 to 1 - FPS drops in a 4870.

Hi there!

Firstly i'd like to introduce myself quickly - i'm jos AKA teamleader, and fed up with manufacturers i started to build my own PC's about 18 months ago. i've had some mishaps (including blowing up a CPU) but that's all part of the fun. im noob, so don't flame on

I have just upgraded my power supply because the old one's fan was playing up, from a 400 (460 peak) Gigabyte P.O.S, to the Corsair TX series 850W psu. This is obviously an upgrade. So i switched everything over and after a couple of odd 'screen not coming on' errors etc, I reset the CMOS, re-sat my RAM and everything worked fine.

Anyway, I was playing some ARMA II and I started to notice something. not only has my average FPS dropped slightly, but every couple of minutes my rate would suddenly drop to 1 for about a 1/4 second before resuming again (this is my fraps counter im going on). Obviously none of this was happening before the new PSU!

This wasn't happening before i put in a better power supply. Have i missed a step - did i need to reinstall windows / my drivers / my life ? I don't know. So i hope you can help me. My initial hunch might be something to do with the memory...

Don't laugh at my odd mix of specs (due to the blow ups when i started learning how to do this stuff).

CPU:e6320 1.86ghz (energy effient)
Mobo: terrible Abit I-45C
Memory: 800 mhz Hyperx (running at 667, see above )
OS: Windows 7 64-bit
Graphics: HIS Radeon HD 4870 ICEQ 4+ TURBO 1GB (FACTORY OVERCLOCK)
HDD: just some standard 250gb cheap HDD. (7200 rpm). i will find out if this is relevant.
I have a small case, but this doesn't seem to have too much ill effect on the performance, with good temperatures as far as i can tell. I have an extra fan for cooling at 100% all the time for good measure.

thanks in advance to anyone that responds, I really appreciate your taking time to help out an idiot