Well, stay away from Gateway, as they are not even really Gateway any more. They got bought our years ago by a cheap computer company called eMachines. I always suggest Dell, but thats my preference since I deal with them on a daily basis and have access to high end support. What you would want to consider is looking into a refurbished Dell from their outlet.
Home class:
http://www.dell.com/content/default.aspx?c=us&cs=22&l=en&s=dfh&dgc=IR&cid=11343&lid=312680
Business class:
http://www.dell.com/content/default.aspx?c=us&cs=28&l=en&s=dfb&dgc=IR&cid=11087&lid=338627
I tell people to go through the normal ordering way of purchasing a Dell, then print out what you would want to order. Go to the outlet listed above and see if you can find the same specs for less money, which 99% of the time you can. You can sort by minimum memory, hard drive, price, etc. I always suggest going with a business class machine like an Optiplex (stay away from Vostro) because here is the difference. Note, to order a business class machine, you will need to select Small Business from the right hand side.
During the manufacturing of hardware, such as a hard drive, they take metriucs on tolerences. They classify the hardware with the best tolerence as "A". Now, you have A, B, & C classifications, and as you can guess C is the worst of the lot. They run hotter and just about meet the speed requirements. C & sometimes B classification hardware is what you purchase from computer stores. Business clas however is mostly A class hardware. This means your computer will run cooler, and last longer. This being said, I always stay away from home class units. HP is good quality, but, again you will need to go to their website for a business class machine to get quality hardware. When you move to business class with HP, the price jumps up quite a bit.
Incidentally, when in the Dell Outlet listed above, the difference in price from a home class machine and a business class machine is very small, if there is one. You will also get the same warranty you have with a new build. And, if you have a prblem with it from the start, Dell will just exchange it for a brand new one. As for dependablity, I see the same amount of problems on new Dell's as I do with refurbs so you really aren't taking an extra gamble on the product.
One thing however, if you wanted a gaming based machine, and you wanted a Dell, you would want to order an XPS which is under the Home class section.