Yup. Get a PC running Windows or Linux. Macs just arent made for gaming.
For 800$ or less i'd suggest getting one custom built.
200$ - Ati Radeon 3800 hd. (GPU)
300$ - AMD athlon 64+ 3.10 Ghz (CPU)
120$ - a GIGABYTE motherboard (pick one that is compatible with your sockets and such)
80$ - 2 x 4GB RAM sticks
100$ - 500 GB hard disk.
If you have more money you can get a sound card, better power unit, and etc.
Also you will need a screen, a mouse and keyboard for a PC.