That single beep is most likely POST. Power on self test. You're board, Asus M2N-E SLI uses an Award bios. The beep error codes can be found at this link: http://www.bioscentral.com/beepcodes/awardbeep.htm
If you are using a single PCI-E card, check that you have the bios set correctly. Shown on page 4.4.6. Do you have you're memory in slot A-1 & A-2? Check that you have PCI-E set for primary display adapter, pg 4.4.4
For windows XP Pro with out SP2, please disale ACPI in you're bios. After SP2 & updates, you can turn it back on. It's realy odd that you're high power PSU did not come with an ATX 4-pin_12v connector for the board. (Needed for CPU).
This is a standard feature for current PSU's. Can you swap the PSU? Considering what devices you are running. You can easely run a 350w PSU? This swap would only be used as a test. Next test, go into the bios & go to Advanced, CPU configuration, check hardware monitor. Stay here a bit & watch temp settings.
Go over these suggestions. Let me know if any one of these suggestions has helped? Can you see the drive in you're bios?