In fact you can install 2 GPU and 2 CPU clients at the same time...but if you are just starting up that can be a lot of work.
It depends on what you have. On vista with a dual core or quad core you can install both CPU and GPU clients. On XP one of the cores will be mostly consumed by the GPU client so you need to have either a very fast dual core or a quad. (The GPU and CPU clients should be installed into different directories)
Right now the GPU client is getting a lot of production done and is very easy to install. Get that installed and then try out one of the CPU clients.
What OS and hardware are you working with? We'll be able to give more specific advice once we know that.
As HY said with your specs we can tell ya the best config.
Many of us that where building farms and buying Q6600s every time they went on sale quickly switched to buying 8800GT/GS and the like. The cost to performance ratio is crazy. Not to mention even when running a GPU client there is still the proc to fold with