A pair of crossfired AMD Radeon 6570s or a single Radeon 6770?

One 6770. Keep power requirements in mind, especially since this is a "budget" machine: one 6770 requires considerably less power than two 6570s.

Additionally, if you can find a 5770 for a lower price, take that. The 6600 and 6700 series cards are simply renamed versions of the 5600 and 5700 series cards.

Interesting proposal, after taking a second look at it, 6570s can be crossfired, although it is rare.

I'd advise a single 6770 over a pair of 6570s though. Dual cards are always prone to problems that a single card doesn't have to worry about. However, the 6570s may beat it in some situations.

Added info:

As far as GDDR5 vs DDR3, there is a nice boost in memory bandwidth with the GDDR5, additionally it overclocks better.

As for the issues you may run into when using crossfire, they are quite common. Adding two cards together by no means guarantees double or even close to double the power of one. Many games wont take advantage of crossfire and you'd be Lucky see a 30-50% increase. Some games however utilize it very well and you can see up to 80% increase. It really depends on what you want to play. However, I've found that it's better to just have one, more powerful, card for these reasons:

1. In a crossfire setup, the chance a card could die leaving you in the dumps is twice as high vs. one card. But I suppose if one were to die at least you'd have something left :P

2. Crossfire itself "especially the crossfire without a bridge which is what these cards use" is very buggy and often doesn't scale well if at all in most games. Additionally, it can cause frame stutter or other visual defects.

3: Dual cards use more power and don't give as much performance/watt as a single card.

All in all, I've tried Crossfire with both a pair of HD 4850s and a pair of HD 4670s and was really let down by the lack of games that utilized it as well as the small amount of benefit it yielded.

Why would you, or anyone consider putting two 6570 cards in X-Fire? If you're lucky, two 6570 cards in Crossfire you come close to a single 6750.

The biggest drawback of a dual card setup is the consistency. In some games you will see an 80% performance increase, in other games it would be a 60% increase, in other games it would be a 20% increase. WOW is a game that see almost no benefit from a dual-card setup.

The other drawback is the heat that's produced by the cards. In my experiences with dual card setups, the heat that each card produces creates a huge hotspot in the case. Only a larger case with good airflow seems to help deal with the problem. As you can guess, the combined heat will decrease the life-expentency of both cards.

Dual card setups do have advantages. let's say you already have a 6770 in your PC. The cheapest thing you could do to boost your graphic perfromance is to add a second card. In some cases, and I stress some cases, it's a better value to buy a dual card setup. There's really nothing that can come close to two Radeon 6870 cards in crossfire in terms of value for the dollar. In other cases, buying two 6770 right off the bat is a poor value. Two 6770 in X-Fire cards would cost nearly the same as a single 1gb 6950, but a single 1gb 6950 performs a little bit better than a 6770 X-Fire setup.

GDDR5 can handle twice the memory bandwidth of GDDR3.

You have to look beyone the amount of shaders unless you're building a Bitcoin rig. For example a 5870 has more shaders than a 6950 but the 6950 performs a little bit better.

the XFX HD 6770 will clock to 900MHz core clock and 1300Mhz memory easily

AMD is ATI crossfire since they merged back in 02