I believe the Dell 5150 will run Windows 7 x64. my advice would be to use it over a 32bit OS. If you have questions, look here;
http://windows.microsoft.com/en-gb/w...#1TC=windows-7
http://lifehacker.com/5431284/the-li...rating-systems
a good way to test if the machine will run x64 platforms will be to download an x64 linux live disk and see if it runs.
try this...
http://blog.linuxmint.com/?p=2489 < scroll to the bottom of the page and you will see a list of servers hosting the 64bit version. select one from the uk and burn it to a DVD. then turn off your PC, pop the disc in and boot from it (usually the DVD drive is set above the HDD for boot priority, so it should automatically run the disc).
NOTE: this disc will NOT install on the hard drive or overwrite the hard drives unless you specify it to do so. it will run linux as a live environment from the DVD. you will not lose any data on your hard drive by running it, however the disc will give you access to the hard drive files. just don't delete anything!!