If you're going to buy a prebuilt, find a place that uses off the shelf parts. Nukem referred to Monarch earlier... They are good and use standard parts.
Choose a motherboard with an nVidia chipset (for AMD), either a 4, an Ultra, or an SLI (even if you don't use SLI). I don't really know a lot about the new 6100's other than they usually have onboard video.
SATA hard drives are the norm now. They are faster than ATA, and the cables don't restrict airflow (very important for keeping it cool).
As for a video card, that depends on what you want to do. If you are only a casual gamer, you could go for a 6800 series card (nVidia). If you are a gamer on a budget (or just want better performance), look at the 7600s. If you are a serious gamer, look at the 7900 series. I doubt you are interested in the 8800s (brand new, mega money).
Crucial, Kingstom, OCZ, G.Skill, Patriot are all good RAM. If it doesn't give you the name of the manufacturer, don't touch it.
Antec, Thermaltake, Fortron (FSP), are all good PSUs. There are many more. The basic rule is that (excluding Antec and Thermaltake) the PSU that comes with a case is junk.
There are many good monitor manufacturers out there. Samsung, NEC, ViewSonic, BenQ, even Dell (their monitors are nice). Do a little research (try checking out NewEgg's lineup and read some reviews). You wouldn't buy a car blindly, so why buy a monitor like that (or even a computer)?