Re: Upgrading specific parts on slowing computer
Your old PC doesn't look "slow" to me, except your CPU has a slow clock-speed. Otherwise it looks "good" - not fantastic. It seems it should run any game on medium settings.
I've upgraded old machines. Adding faster CPUs, more memory has had very little improvement to my games. Adding a much faster graphics card had a very substantial improvement.
If your older PC has PCIe video slot, I would suggest spending your money on a really hot new card. Just make sure your PSU can provide adequate power to avoid crashing and spontaneous reboots.
The Gateway you mention looks just as fast as your older PC.
Definitely x64 is the way to go. Ditto about "too bad it doesn't have Win 7.
As a PC tech, one of the things I habitually do is clean up junk files that collect on the hard drive. It would be too long of a reply to list all I clean up, but I would guess that's why your older PC has slowed down. Too many start-up items, too many temp files, too many uninstall files/folders, etc. Clean the Registry and defrag your Pagefile and defrag your Registry, etc. Any one thing has little affect but all together makes a very noticeable difference in speed.