4890+4870 Xfire questions

[NOVAA]

I'm going away to college this summer and have been planning on building my brother a gaming rig so he can play when I take mine with me. Budget has been loosely laid out as around $700, and thats including my current 4890 I'm going to give to him when I get a 5870 (Don't need one really, but I sure want it and "He needs a graphics card" justifies it for me lol)

So with this budget, I've been thinking of ways to give him a bit more horsepower. CPU will be a Phenom II x4 (Probably stock or minor OC on stock cooling), whatever MidATX case is on sale at Newegg at the time, etc.. Should be fairly cheap, so I think I have a bit of money to burn. Recently, thought about Xfire 4890's, but I probably won't be buying for a couple more months and they surely will be gone by then. Figuring I'll probably be able to find a 4870 still, and its a good $40-50 cheaper, began to consider getting the cheapest 4870 and combining it with an ASRock motherboard.

Couple questions, performance with this setup is very good right? I've heard that the 4890 will clock itself down, and I've heard that it stays stock speeds and takes on a bit more of a role, making a 4890/4870 setup faster than a dual 4870 setup. Which is right?

Secondly, I've never run a dual GPU system before, what kind of PSU would I need for it? I've heard some say you need like 800 watts which seems a bit much to me, and then others say that a quality 600w system is often better than a more midrange 700-800w supply.

Thanks guys

[GotNoRice]

4890+4870 will work fine, but will only be as fast as dual 4870s. The 4890 will clock itself down. This is also the case if you had two 4870's with different clockspeeds. It's always the lowest common denominator with crossfire. That being said, 4890+4870 is a configuration approved for crossfire by AMD.

I ran a single 4870x2 and my overclocked quad at 4.4Ghz with an Antec Earthwatts 650 for a while.

[NOVAA]

4890+4870 will work fine, but will only be as fast as dual 4870s. The 4890 will clock itself down. This is also the case if you had two 4870's with different clockspeeds. It's always the lowest common denominator with crossfire. That being said, 4890+4870 is a configuration approved for crossfire by AMD.

I ran a single 4870x2 and my overclocked quad at 4.4Ghz with an Antec Earthwatts 650 for a while.

Yeah thats what I figured I was just hearing other things. I would want to do twin 4890, but I doubt I'll find them when I need them. Its not like twin 4870s vs. 4890's will make a difference to him, he'll forget to enable AA in his games lol so I doubt he'll care about that.

And as for the PSU, I think I'm going to go with a high quality PSU in the 650-700 watt range, GPU's won't be OC'ed and neither will the CPU so I'm sure it will be fine.