I think he meant "hardware" and that's a lot of information to collect. I haven't done my research, maybe if we summon Asus and if he's feeling really nice he might drop a dense wall of enthusiastic text on you, but the question is very general and it would be good to know things like:
- Price range - best money can buy? good value for price?
- What features do you want it to have? SLI? Dual monitors? Surround audio? things like that
- OS you want to install on it
Definitely have to answer a lot of questions about the machine you want before you begin shopping for hardware. You have to be much more specific than "custom gaming PC"