You can try
Download STEAM repository
$ su -
# wget http://spot.fedorapeople.org/steam/steam.repo
Move STEAM repo file into your system repo...
Add more ram, possible get a faster hard drive 7200 rpm. Or just get a cheap ssd, you will see a big different !
Do clean install with xubuntu, or...
Its quite simple from this article http://namhuy.net/903/how-to-upgrade-linux-mint-14-to-linux-mint-15.html
sudo sed -i 's/quantal/raring/'...
Separate names with a comma.