Litecoin mining is the most popular
but there are many other scrypt based coins:
Feathercoin, Worldcoin, digitalcoin, Megacoin, to name a few.
If you want an overview of PC building start here.
Want to get a ton of GPUs with a single motherboard? Many people don't know you can use PCIe extenders to hook PCIe x16 GPU cards up to a PCIe X1 slot. Many motherboards have multiple PCI Express ports, some have as many as 5. Remember to get two power supplies or more for more than two GPUs! At this point in time we also recomend that use FM2 socket motherboards for the new A-Series AMD cpu. It will save you money over the Intel versions.