PC slows down after running a game for a short time

Hi. I have a Pacard Bell I-connect 1200 with a GA-6WMM7 intel 810 celeron motherboard. 3 PCI sockets one of which has an nvidia G-Force FX. 5200 PCI card. This is an improvement on the old Voodoo 4 card I was using before.
My problem is that after a few minutes of playing games the whole thing slows down, I had this with the voodoo card but not as bad.
I have also upgraded the Ram to 256 mb. I thought these two changes would improve this slowing down problem.
Is there someone out there who can help me identify why this may be happening?
Please help.

What operating system do you have, very important. If you have XP, 256Mb of RAM really isn't enough at all, when I had 512Mb of ram, leaving my system turned on for 12 hours doing nothing caused it to slow down incredibly when I came to use it. With 256Mb, playing an memory intensive game, it will sooner or later start looking at the hard disk for necessary data, which will cause the slow down.

I had that problem too, not just when playing games but after 6 hours or so of general use/ idling it would be a fair bit slower than before.

Solved when i went from 256MB RAM upto 1024MB

Thanx, my os is windows ME but I will be going over to XP ASAP. My motherboard will only accept 512 mb of Ram (apparently).

Is there a motherboard you would recomend (168 pin as i've heard memory is going up in price) preferably Pentium that I could upgrade to?

well i think it would be better to build a new pc , because i think that celeron will probably affect the increase of speed if you get new ran(if im right in saying so).

Cash flow is a problem at the mo

I would say that you'd be better off with a new system too, get an AMD athlon, 512MB RAM etc and it'll last you a fair while

Obviously some parts will be ok from your old system so save up and buy lots of new bits