upgrade necessary?

Hey there.

For an upgrade I'm lookin at: A64 3500+, DFI lanparty mobo, 512mb DDR 400, and a X800 Pro.

I only have two hard drives. My PSU is: http://www.enlight.co.th/PSU340.htm

Will a new PSU be absolutely necessary?

I would get a new PSU. From the specs on your current PSU, it probably won't have enough power for your CPU and graphics card, especially if you're planning on doing any kind of OCing. Your 12V line probably won't cut it as your CPU and graphics card are heavily dependent on that line. I would look for a PSU that supplies atleast 20A on the 12V line or a PSU with dual 12V lines. I would suggest picking up a Fortron BlueStorm PSU. They're very good PSUs and are reasonably priced.

Yeah, I think it would be a good idea to get a new one. That is cutting it too close for comfort.

any specific x800 PRO?