The fastest Quad core you can use, is the X5365. As per the 32 bit aspect, that system uses EFI32, which restricts you to the 32 bit Kernel of OS X (MP 1,1 and 2,1 systems are EFI32 = '06 and '07 systems). '08 and newer use EFI64.
Currently, OS X has both K32 and K64 (both 32 and 64 Kernels), and K64 CANNOT work as you need EFI64. There will be no update to the firmware to make it EFI64. Worse, OS X won't support dual Kernel versions for long. It will disappear in either 10.7 or perhaps 10.8, as Apple's already announced that they won't keep it going (they don't like interim products, but didn't state which version would be the last to have dual Kernels).
It also is limiting you on graphics card choices, as the newer nVidia models are EFI64 only, while the ATI products are currently using EBC (EFI Byte Code), which allows them to work in either EFI32 or EFI64 based systems.
So long as your software will do what you need in 32 bit Kernel version, you're fine that way. ATI will likely continue to use EBC, so you may be able to continue upgrading graphics as needed before your system hits EOL for you.
Also, that system will run 64 bit Windows or Linux just fine, and under either of those operating systems, any PC PCIe graphics card will work, so long as there's drivers for the specific OS version used.
Odd that it's getting to the point it has more life as a Windows/Linux box, but there it is.