A 5770/6770 is about the largest card it would be advisable to use with that power supply.
The 5750/6750's really aren't worthwhile because the 5770/6770 is basically the exact same price. Here's one for $90 after the rebate; SAPPHIRE 100328L Radeon HD 6770. (There's one 6750 for $85, but the rest are $90 or more)
It should be fine even if it doesn't. 6770's don't draw much power. I believe a version of the 5770 (which is the same care) was actually made w/o any 6-pins at all actually.
The i3 is a very low power CPU, he shouldn't hardly break 200w used under load, which leaves plenty to spare even if his power supply only delivers 75% of it's rated capacity.
Yup they do have a 6 pin free card. Your board needs to be newer as older ones limit to 75 watts for a pci-e slot it self.
On the topic of power, yes, i would bet well under 200(gonna guess under 150 for the most part), but playing it safe with the unknown power supply(age, will it kill it self without killing the pc?) here.
Check my "Member configuration" for my media center
A 6750 and 6770 will borh work fine. Nuke is just being extremely overcautious. 6750's and 6770's as i said before are literally the same price, but the 6770 has 800 shaders @ 850mhz, while the 6750 has 720 @ 700mhz.
The 6750 is just a bad value card right now. The power use is very close to the same between them as well, there is just no reason to buy a 6750 period. You'd get ~10-20% worse performance for the same cost.
Yes your power supply isn't great, but a 6770 will at most push it to 50% of it's rated capacity. That it well within the range of safety. The only way that would cause a problem is if you just had a bad power supply, in which case it wouldn't matter what card you got at all. As long as you don't have a genuinely faulty PSU, either of those cards would be completely fine to use. But like I said, they cost the same while the 6770 is straight better.
Get what you want, but you're literally throwing away free performance if you buy a 6750 as opposed to a 6770. And at that price level the performance will be noticeable, we're not talking the difference between 110 and 120 FPS here, but something that could well decide if a game is playable or not on semi-decent quality settings.
On average there is about a 11 watt(20 watts if you want to run furmarks) difference from a 5750 to a 5770. So yeah, if you are that close to the power supplies limits, either card is bad.
The 5770 is the only card that makes sense to use for you.
I am not going to lie, I have not used your power supply(thus the being extremely overcautious) and do not like its older design(as it is at best like a modern 300-350 watt unit anyway, so i would consider about 250-300 being its top end power output. Do not be fooled by overall power, its all[ok, mostly] about 12 volt these days).
Personally i am running a 5770 + i5 750 on a 300 watt power supply without issues at all(and never pulling over 200 from the wall(Prime95 + Games) most games are in the 140-150 range. The idle is under 40watts)
Can i make a promise on a power supply i have never used? Nope.
If you are worried, or want to know just how much power the system takes, pick up an energy meter(it may surprise you), it will tell you how much power you are pulling and DC power is lower since there is a loss swapping AC to DC. Just don't pull much over 250 from that thing and you should be fine.