Personally I like the single GPU solutions for these reasons:
* Don't need added PSU power. (saves $)
* Not using more power. (saves $)
* Better driver/game support (saves time/efforts, which saves headaches)
* Mobo requirements not as critical (more options which means you'll save $)
Reasons SLI/X-fire is good:
* Better performance at higher resolutions (most of the time)
What was your purpose of the build??
What are the other parts of your build??
In the $250-350 CPU price, there isn't ANY AMD CPU in that price category, so Intel would win in that price. Depending on what your doing with the system, AMD can be a good system to have, but it all depends on your needs.
The i7 920 (s1366) is probably a good enconder/SLI/X-fire system to have, but it's a bit spendy, IMHO. Some who will use it to it's limits will prefer it the other selections, but I think it's still too much $ (I like a good performance/$ ratio).
The i7 860/750 (s1156) is a pretty good option right now. I don't think the 860 is that much better ($80 difference at Newegg), but that is just my opinion, especially if you OC a bit.
The Core 2 Quad is on a dead socket (s775). It is cheaper than an i5 750/i7 870, but I don't think it's worth the $, especially if your building from scratch. You'd be using DDR2 (which isn't bad, but isn't the standard now).
I like the i5 750/i7 870 for the throttling/power saving/auto OC'ing that the other 2 don't have, but that is just me.
So in summary:
I'd probably go with the i5 750 and use the $80 for other upgrades or for savings for a later upgrade. I don't know if there is going to be any CPU upgrades on the 1156 socket, but it still seems to be good buy at $200.
In the $200 CPU price category, I prefer AMD 955 Black Edition CPU. I like/prefer it to the Intel platform because of the possible longer lifespan and possible future upgrades on the AM3 socket. The 955 Black Edition OC's really easy and doesn't cost you anything!! 1 min. in the BIOS and your up to 3.6-3.8 gHz. You have to a bit more to get the Intel in that category, but they can OC a bit more than AMD, with the right knowledge.
AMD Phenom II X4 955 Black Edition Deneb 3.2GHz 4 x 512KB L2 Cache 6MB L3 Cache Socket AM3 125W Quad-Core Processor - Retail http://www.newegg.com/Product/Product.aspx?Item=N82E168...
I like the single gpu solutions where possible, for all the reasons that lunyone stated. Only if you will be gaming at 2560 x 1600 might a multi card solution be reasonable.
The 5850 is a good card for gaming.
All of the <$300 cpu's are good and will drive the 5850 well. They are good enough to drive even a 5950.
How much do you need many cores? Very few games can use more than two threads.
The i7- 920 and i7-860 give you 4 cores and 8 threads. They cost about $290. they are clocked at 2.66 and 2.8 respectively.
The i5-750 gives you 4 cores and 4 threads for about $190. It is clocked at 2.66.
There is a new option that you may want to consider; the clarkdale 32nm series.
You can get a i5-660 for about $210 clocked at 3.33 it has 2 cores and 4 threads.
For $300 you can get a i5-670 clocked at 3.45 with 2 cores and 4 threads.
If you are willing to overclock a bit, go with the lower priced chips. A 10% overclock is trivial with any of them, giving you the same performance as the highest priced part.
Even without overclocking, any of the parts being discussed will drive the 5850 well.
With your budget, I think I would go the i5-750 route, and use the $100 savings to upgrade to a 5870 for gaming.