Quad core Xeon basically = i7, costs the same. I guess that answers your question.
If you would build it any cheaper, you would end up with an i5 and i3, but it would be more expensive than an i3 or i5 iMac. You would need another motherboard with another socket, means another factory line, means more cost for Apple, because nobody would buy that. Teh l33t g4m0rz will still cry out that you pay $1200 too much when it gets sold for $1500, and the current Apple-users which were interested in such a machine will more likely get an iMac, because it contains a $1000 screen with a $800 i7 computer at $2200. Get an iMac, a Radeon HD 5750 should be sufficient anyways. i3 with 4-way SLI doesn't make sense, IMHO.
This whole "but the MacPro is more extensible" is BS, because the only way you can use PCIe on a Mac is by adding graphics or RAID - or SATA, USB and FireWire ports. There are no Soundblasters, video capture cards or what ever you would put in a PC. PCI on a Mac got replaced by FireWire, not by PCIe and USB like they did on PCs. SATA-HDDs can be daisy-chained on FW800, ya' know, and the internal bay of an iMac can hold 1 3.5" 3TB hard drive plus an SSD. Instead of the optical drive, you can do a little modding and add an eSATA-port to the iMac, where you can hook up your RAID0+1 or what ever. SATA for DVD-Drives is overkill, a $15 USB 2.0 enclosure will do just as well.
If you care about FPS in Windows games, build a Windows machine. A Mac is a boat in the Mississippi Delta, maybe slower than a car, but it gets you faster from A to B. What you want is a sports car. Don't buy a boat if you care about MPH.
You want a Mercedes
ALK (i5) when there is only SLK (4+6 core) and SL (8+12 core), but a
Volkswagen Eos (overclocked i7 PC) is more fun with more power for less money than the CLK (iMac minus the display) costs - but they don't do SLK nor SL.
And so on, and so forth…
