There isn't any sense in that ... you could build a machine with 2 dual-core Opteron processors on a server board with the NF4 Professional chipset, grab a few gigs of DDR or DDR2 memory, a good power supply, and a couple of GeForce 7900GT's run in SLI, along with near a terabyte of storage and a double-layer DVD burner for less than you'd pay for a much weaker machine from Apple. If you don't happen to have a dozen XP licenses lying around the house like I do, you could still fit a copy of XP Pro under that price tag. You'll have a machine to be reckoned with. There's literally no computing task it wouldn't be up for.
As for how many PCIe x16 slots it has, that's largely irrelevant. What matters is the number of lanes of PCIe connectivity and how they are divvied up. Modern high-end boards for standard PC systems have 32 lanes of connectivity for graphics alone (normally split into 2 16-lane slots in SLI or Crossfire configurations for dual videocards), and less beefy ones have 16, yielding 8 lanes per slot, (8 per card). This is also more than enough bandwidth to accomodate 2 dual-GPU cards like a pair of nVidia's ridiculously-overkill 7950GX2s. You can have all the SLOTS you want but what will a third slot net you? The ability to connect a FIFTH monitor? You can already add loads of monitors if 2 or 4 really isn't enough for you with PCIe x1 or x4 cards from companies like Matrox-- or just standard PCI graphics cards that can be had on eBay for $10 .. in packs of 5.
Most boards still have 2-3+ standard PCI slots. And you cannot add a third video card for 3-card SLI/Crossfire, nor a dual-GPU card for 6-GPU SLI .. there is support for it.
So tell me, what reason is there to buy that Mac ...