Those Optiplex's are being phased out after workhorse service in schools and libraries.
Googling GX280 + RAM Crucial says it takes 2 gigs in 4 slots @ around $20 per gig, so you could spec it carefully to upgrade then test install Windows 7 with a gig with the option to upgrade another gig later if you like it.
You'll need at least a gig of RAM to run Windows 7. I've run with 512mb before and couldn't run gadgets or even an AV. Windows 7 is already so lean it will default to run on the lowest hardware specs, but 512 is just asking too much.
Once I upgraded to a gig my old HP 510 beach laptop runs fine without hesitation with it's Celeron, and a Pentium4 2.8ghz is a better processor, so it might be doable if you're wanting to experiment.
Thanks for your reply!
I don't really want any eye candy on the machine,
Or anything else like Gadgets or Aero.
But I will need a Antivirus because I don't know what he is going to be looking at.
Am building the computer for my dad.
It's kinda like his Christmas present.
The computer is just mostly going to be used for online shopping and watching a lot of YouTube videos.
So I don't really want the machine to be fast or anything.
Am almost done with the computer,
I just need a Wireless adapter for it,
I have the router in my room.
He is going to have his own computer in his room.
Since it's a desktop, it needs a wireless adapter.
Although am not sure which wireless adapter I should get.
I think I might try a dual boot for the sake of experimenting!
If it doesn't work, or if it's any more slower that Windows XP.
I'll just delete the dual boot and stick with Windows XP.