whats a good version of linux
 

for hp vectra vl400 dt pen3 500mbram with old monitor

I've had various versions installed on a P3 with 320M of RAM. I think anything in the top ten on the "hit list" at distrowatch should be fine.

I think some old version ubuntu like 5.10 is good choice

I disagree---a current version of Linux is more likely to support a random piece of hardware--old or new.

Old is bad, no updates available, no securtiy fixes available, definitely use a current version of Linux.
if performance is an issue switch to a lightweight desktop or window manager.. XFCE, fuxbox, or something along those lines

I found Debian to perform fairly well on PIII hardware.

XFCE the Other Desktop

check out anti-X
 

Anti-X is by far and away the best distro for older hardware. You can get versions with XFCE, fluxbox or a light version of KDE

http://antix.mepis.org/index.php/Main_Page

i had a bit newer configuration, but i used debian a while ago and it was quite ok.

i use slackware on older computers, so this is what i can reccomend to you

Debian