It all depends on what you're looking for in a system.
If you want an average system, used mostly for browsing and other petty tasks, pick something out with a Celeron processor or an AMD Sempron. They do the job, and they come at an extremely affordable price.
If you want to build your own system, my recommendations are AMD processors, PNY Technologies RAM, Gigabyte motherboard, Western Digital hard drives, Sony CDROMs, Logitech keyboard/mouse, Creative sound card, Nvidia video card, etc., etc.
However, I think you're better off with a bit of a souped up pre-built system. Gateway, as of now, is not a good choice. With what I've had to work with on their newer models, I'm not very pleased with the results. Dell, on the other hand, has improved drastically, and make some pretty solid machines. But, I would recommend to you the use of an eMachine. They're quiet, efficient, and relatively cheap.
The eMachine that was purchased for me at work was for about $500. Has an Athlon64 with 512MB of RAM, 200GB, and up to 128MB shared video card (ATI X600). Overall, gives me all the power I'll ever need.
If you are getting a pre-built system, I'd suggest one with more than 512MB of RAM, mostly because of that shared memory issue. Also, I'd make sure to get it with a PCI-Express slot inside, just in case you want to do some upgrading to your video card.
So yeah, eMachine would be my first choice. Dell second, and homemade system 3rd. If all else fails, you could try Gateway, but don't get your hopes up if you want the system to function normally.
"God help us. We're in the hands of engineers."
- Dr. Ian Malcolm, Jurassic Park