You won't get any version of Windows, beyond 98, to run with 8Mb of RAM - presumably you mean 8Gb.
Yes, there is a 64-bit version of XP. It's OEM only, and probably not available in shops nowadays, but you can normally pick one up on eBay for about £60 ($100). Be aware that not all software will run happily on XP64, and it doesn't support as much hardware as the 32-bit version. You may be better of with a 64-bit version of Vista. Or there are 64-bit versions of Linux, BSD, etc.
Be aware that there is also another 64-bit XP designed for the Itanium processor - you don't want that version!
I run XP64 and it is fine, but I don't have any unusual hardware or software. Do you have a particular application that needs 8Gb of memory?