First, get an AMD, or Intel processor, and motherboard. Put those in, and get a new big HDD. Newegg has a 500gb for under $100. Put those in, then get ram compatable for your system. The get a can of KRYLON spray paint. Cover up any unsightly Crapple logo's.
Amd Athlon 64X2 @ 2.8ghz or better, or a Phenom procesor however you can get it.
For intel, get a P-4 dual core @ 2.8ghz or higher (Socket LGA 775), a core 2 duo @ 2.8ghz or higher, a core 2 quad however you can get it.
Make sure to get a motherboard with a compatible socket, and note the type of ram it takes. Put in at least 2, but preferably more gigs of ram.
Then, finally for God's sake don't tell anyone you bought an apple, or tell them that you did and gutted it to make a real PC.