I take it you have an older system then. Anyway you would probably be better off with a single GTX 275. Even though two 250s in SLI are generally faster, SLI does introduce some CPU overhead so it is possible that on your system two GTS 250s will be even slower than a single 250 if your CPU simply can't keep up. Depending on your CPU and Resolution then you may not even get much benefit out of getting anything faster than a single GTS 250
megaman, he said that he is making an all new system.
Also, to the OP: Get a single 275. Performance difference is not that huge and you dont get the SLI related problems like stuttering, heat issues etc. One powerful card is always better than two lower cards if the performance difference is minimal.
BTW, what are the other components are you thinking? Maybe, we can help you there too.
I just ordered the parts yesterday, lol, I got a core i7 920, asus p6t, and 6 gb of ocz platinum ddr3. I stuck with the single GTX 275 and got the evga superclocked edition.
My current system (actually I haven't used it in about 3 years, long story) is an FX-53 with 1gb of ram with a 6800 ultra. Once I land back stateside, I will benchmark the sucker and then scrap it for parts.
I have always kinda been against SLI because of the cost (dollar wise, electricity wise, and heat wise) but the benchmarks of the gts 250 in sli looked a little tempting.
Actually it's kinda funny that everyone here told me to be careful about SLI or not go for it. Over in the new system builders forum everyone and their grandma is touting off the wonders of SLI and it started going against my better judgment. Micro-stuttering was a big worry aside from the heat and electricity.