A friend of mine recently upgraded her computer and gave me her old graphics card. This card is very similar to my current one (a Radeon HD5770 and a Radeon HD5750) so I'm hoping to run both cards in a Crossfire setup, if my PSU can handle it.

My PSU is a 4 1/2 yr old Corsair TX650. In addition to the 2 GPUs, it would need to power an Intel i5-750 CPU, 4 sticks of 2GB DDR3 RAM (though this could possibly be swapped with 1 8GB stick, if that would make any difference), 3 7500rpm HDDs and 1 Blu-ray drive (as well as a few case fans, of course).

I've tried consulting a few different calculators. Some of them gave me minimums that seemed very low (350-450W). This one gave me a 600W minimum when I listed my system as having 2 HD5750s and a 650W minimum when I put in 2 HD5770s. Corsair's PSU finder included the TX650 when I told it I had 3 hard drives, but when I included a 4th (in place of the blu ray drive, which they don't have a field for) it started suggesting 700/750W PSUs instead. The AMD website suggests a 600W for 2 HD5770s in Crossfire mode.

The impression I'm getting from all this is that it would be just about enough, but possibly cutting it a bit close. But I know it's possible that some of these calculators overestimate requirements to play it safe. But then there's also the age of my PSU to consider. And I don't really have much experience to judge from.

I've done the research, now I'm just looking for another opinion or two. If it was you, would you feel comfortable with this setup, or would you consider it too risky?.

I would make sure you can use 2 dif. cards in crossfire mode before I would put the 2nd one in my computer if I was you.

Yes from what I have seen on this the higher W. you have the better they will run, I always go with one that is 100W higher than what is recommended.

My motherboard supports crossfire and the chart on this page seems to suggest that the HD5750 and HD5770 are compatible. They're very similar cards, the HD5770 is really just a slightly better version of the HD5750. The only potential issue I can find is the PSU. If it's not good enough then I won't bother, but seeing as I unexpectedly ended up with a pair of compatible cards, I thought it was worth investigating the possibility.

Look for reviews for both the 5750 & 5770 on reputable tech sites (like Anandtech, hardocp, techpowerup, hardware canucks). Go to their power consumption sections and find out the highest power consumption of each card (card only, not total system draw). Add the highest value you get for the 5750 to the highest value for the 5770.

Go to the intel website and look for the max tdp of your processor.

Add that to the value of your cards combined power draw.

Take the total and divide it by 12.

Thats the minimum number of Ampère your power supply should supply. Under no circumstances should you try with less than that. Ideally you should have that number +25%.

Then Google your tx650, find out how many A it provides on the +12V rail and compare. You can also just look at the sticker on the side of your tx650 and look where it says "+12V".

That is really helpful advice. Thank you so much!

I checked them out on all of the sites you listed. Hardware Canucks gave me significantly higher figures than any of the others, which I thought was strange, so I went back and double checked, turned out their figures were for the whole system, so thanks for mentioning that as a thing to look out for!

The minimum +25% figure I got is 31.77. The figure in my PSU manual for the +12V rail is 52A. I take it that means I'm fairly safe? (Interestingly, when I ran the first set of figures (from hardware canucks) the minimum figure I got was still below 52, though the +25% brought me up to 61. Does that seem possible or did I screw up the calculations somewhere?)

No 31.77 is fine since if you took the other values you would be counting the power of the processors that canucks used for their test twice, plus your own.

I think you should be fine with 52A. PSUs do get used over the years but it seems to me you have enough room for error.

Great! Thank you so much for the help