Loooong time reader of the articles here, but first time poster.
Some very short backstory. I've recently come back into posession of an older gaming machine I built for a family friend. He didn't need it anymore, and decided I should have it back. It's dated, but a very nice performer considering it's age. It's a Core2Duo 6750 2.66 in an ASUS P5N-D, with 4GB of admittedly cheap ram and a 9600GT 512MB. Got it back, and it was working fine, played some games, felt some nostalga, all was good.
Then I found another XFX 9600GT of the exact same model in yet another old machine I'd built, and baby it was SLI time. Results have been fantastic! Some games get amazing fps at 720p, even with all the settings up and FFXA on. Fear 3 and CS:Global Offensive run just like they're being played on a new computer, north of 80fps at all times. Other newer games like Deus Ex: HR run at around 40fps with everything at max. Overall, I've been seeing amazing performance in games that aren't CPU bound, and decent/eh performance in games that are. I've been keeping tabs on temps/usage on my gpus/cpu with msi afterburner's OSD combined with HWmon32. I figured I'd give OC'ing a try, since I really won't be too heartbroken if the thing goes up in flames even if I bone it.
Thing is, I'm hitting a few snags that I'm not sure how to diagnose. In Fear 3, I get about 80fps, with both GPUs pulling their full weight at 98% use, with CPU use around 60%. And from time to time, both GPUs will drop to 50% use and CPU use will scale back some too, and fps will drop to 40, goes in and out. Seems to happen once things warm up, but temps are south of 55C on the CPU and south of 75C on both GPUs. I've also had Deus Ex BSOD lock on me, with the nVidia driver being the callback. The system wouldn'tc get video again until I removed one of the GPUs and turned off the overlock.
There are a few things I can think of. Maybe I don't have enough power being one. Currently I've got a 430W PSU, which worked fine for just one GPU, and worked fine before I started OC'ing the CPU. Could it be that simple? I'm having a ton of fun playing with this, and would hate for it to go bust over something so pedestrian. Normally I wouldn't be so uncertain, but I don't remember how much PSU was "enough" back then, only now. The PSU only had one PCIe power connector, I had to use an adapter to connect the other GPU. How much PSU do I need for this?
one of those 9600GT's take 180 watts max, if you said you have 2 of those and your psu is 430watts then you deff need more power, id recommend atleast a 600 watt corsair/seasonic for now but if you have the dough get a 750/850 watt corsair/seasonic for the future. dont cheap out on the psu and get a 30 dollar 600 watt psu because those are trash.
With thr Pc being as old as you nsay you will be better off getting a low oriced psu since your not into the Pc for the long term and with you having as much fun as you say your having it won't take long for you to start wanting a new build , something that will last a while.
You can get a $50 psu at Newegg or Amazon that while it is inexpensive will last for quite a while and you can transfere to a new computer if you decide that you want to build a new one.