I have an MSI K8T Neo, Athlon 64 3K+, and Antec 430 True Power. My experiences trying to use it in that box:
Uninstall 9800Pro and drivers.
Install 5800, plug in to hard drive power connector. I've had the following results:
1. Sometimes nothing happens, no beeps, black screen.
2. Sometimes it posts it's bios, but the letters are missing pieces and the screen has other anomalies.
3. I actually got it into Windows XP a couple times with much garbage on the screen. Once I was able to try to install the Siluro drivers, it recognized it as a Ti4600. (inspection of the card/chip verify it's a 5800)
Installed Ti4200, no problem, install latest nVidia drivers. Put 5800 back in, repeat of above with exception I never got a usable desktop.
My four year old has a 300W, nForce1, 1700+ rig. I put it in there, no beeps, nothing.