Lias: If you want custom built, you pretty much can't go past Computer Lounge
Absolutely! I'd steer clear of PBTech, and while I was happy with the build I got from Playtech I wouldn't use them again due to support issues when the motherboard failed two years (and still in warranty) later. I've never had a problem with Computer Lounge.
Sorting out what you want in a custom build is as simple as asking on the forums here, and/or on the Computer Lounge forums, for recommendations to suit your particular needs, doing some research online, then looking at getting the most bang for your buck. The biggest advantage of a custom build over a pre-built branded PC is upgradeability/future-proofing. A little planning can get you a PC that will last for years.
It may seem daunting if you have always bought a brand PC but those are built to a "one-size-fits-all" and are governed by price. Custom builds give the options to have a system to suit needs, and give the option of going from a basic PC to an all-powerful rig in stages as a budget allows. Computer Lounge also has a thread on the forums listing their build deals. These can be a good starting point to work from or you could just tell them what you use the PC for, what you hope to be able to do with it in the near future, and ask for recommendations and a price.