If you look on the back or side of the tower there should be a Service Tag number on a sticker. If you find that and enter it into the service and support section on the Dell website it will take you to the correct page for that model Dell. From there you can download the correct drivers, manuals and look up the specs, etc.
If you want to try before you buy you can install windows 7 and leave the product code blank. You can then run it for 30 days as a trial, longer if you rearm it. So if you know someone with a Windows 7 32 bit DVD that you can borrow make yourself a universal install disk and you can try any version you want.
USB Windows 7 Installation Key Drive - Create
Windows 7 Universal Installation Disc - Create