A 2405FP (or other 1920x1200 LCD) should be able to be driven off a well made single link DVI card. The monitor DVI port is only single link, so going dual link will not do you any good.

If you go with a PCIe solution for graphics cards, make sure the motherboard can actually deal with multiple graphics cards in a non SLI setup. Some may offer the 2 16x slots, but only work right with using SLI to a single monitor.

Apple's new PowerMac G5 systems do officially support up to 4 PCIe graphics cards with up to 8 total 23 inch displays (or 4 30 inch displays). Price wise,it might be the way to go since you don't need to worry about motherboard compatibility this way. Though if you go the Linux route on these, make sure support for the new system is available first, since the PPC Linux side tends to lag behind the new Apple hardware with radical changes in system design. Since no Mac before October shipped with PCIe, it may take a little bit of time.