Building gaming PCs for profit, and accepting payment?

I need some extra cash so I'm considering building PCs for local clients.

I already have most of it planned out:
For the price of about $1000, I could build a sleek-looking machine with a Wolfdale Core 2 Duo, plenty of RAM & HDD space, DVD Burner, XP Pro, and a nVidia or AMD graphics card that would dominate in Crysis -- while making a $150-300 profit.

I would advertise only locally, using mainly Craigslist.

I would also include Anti-virus, anti-spyware, productivity software, and configure XP to minimize resource usage. I would deliver the PC to the client's door and maybe even help them set it up.

I would make it clear that I can only provide warranty service on parts, not if they load their PC with spyware.

The only speed bump I've run into is how to accept payment. I would require it upfront to pay for the parts, but how in the world am I going to convince anyone to trust me with their money, given the fact that they won't get their new PC for 1-2 weeks? Any suggestions?

You may be able to set up an account with an online dealer such as Tech Data, Newegg or TigerDirect. If you can set up such an account, then your purchase terms (Net 30, Net 15, etc) would allow you to order the parts, assemble your machine and sell to your client. Provided you can get your money from the client before your account terms are due for those parts, you can effectively set up cash flow that would not require you to spend money out of your own pocket.

You have to invest something in your new business ; in your case it seems it has to be the price for the parts .
if you have some wanna be clients and you are confident about yourself buy the parts and make the rig in a couple of days.If (s)he doesn't buy it and it's worth the price you are going to sell it elsewhere .

I agree, you need to build the computers first then let the customers buy them knowing exactly what they are getting in the computer. that was also something i had considered doing for myself. I already build computers for friends and family that will give me the money upfront but have not tried to go into doing it for other people. Busniess license and insurance would be needed from what I was told in case somebody wants to sue you for lame components or anything such as that. doing it for friends and family I do not o it often enough to make any real income of it to need such things.

I build sell and repair computers and its tough at first. After a while you get repeat customers or people who have your computers send their friends your way. Don't expect to make $300 on a $1000 tower thou. No real money building low-end rigs nowadays. I specialize in high-end towers and don't even monkey with $1000 towers. The money today is in repairing them, not building them. The fastest video card out right now, the ATI 3870X2 doesn't even "dominate" Crysis and the card costs $450. How would you sell a gaming rig that would "dominate" Crysis for $1000 and make a $300 profit?
If you don't have the product to sell nobody's gonna front you the money to build it--too many established builders out there.
I suggest you save some money if you plan on going into the business, dont expect anyone to front you money because --they won't. Also, be more realistic in what profit you expect to make. 30% profit simply isn't going to happen, more like 10%.