As Evorobin says, before you go looking for a vendor, you need to define what you want to use the machine for. I would say, think about how you use your PC now and how it might be used if it were more powerful, then add a bit more on top! - It's amazing what new uses you can find for a PC that you never even thought of. Basically, to get the most long-term out of your machine, try going that extra yard and spending towards the top end of your budget, but don't go mad.
There are lots of considerations, including noise and power use. A good parrallel here is what you consider when buying a car. Do you want it for practicality, economy, price, or speed, or a certain combination of them all. Sure the most powerful computer can do the lot, but these days the most powerful computers are very Power hungry. It's like running a hair dryer all day, every day - would you like to pay the electricity bill for that?
If you only want it for home accounts, office type applications and browsing the internet, you will be happy with a budget machine. It will also be quiet and use minimal energy. If you want to play music, DVD's and video, then you need something with a medium sized hard disk and a half decent graphics card, but you are still looking at the lower end of the market.
If you play 3D games occasionally consider say 2 models down from the top notch where you will get best value for money and get respectible longevity out of your machine - These machines might be a little noisy when gaming, but are otherwise not so bad.
If you like the idea of checking AdBrits while sitting out in the Gazzebo? - Consider a laptop and wireless internet connection.
The top end machines are for the crazy out and out gamer and there is a BIG price premium. You are looking at a top spec machiine with Major cooling, a set of ear-muffs (or customizing to reduce the noise) and a noticeable bump in your electricity bill!
In terms of specific machines, or specific manufacturers, I would say, hold out until you see a computer magazine with an article bench-testing a selection of computers in your range. $8 might seem a lot for a magazine, but it's worth the investment.
One vendor not mentioned up to now, is to buy direct on-line from
Dell. I think they are a fairly reasonable yardstick price-wise. They are generally in the top third of the speed scale for a given range and quality is good.
Whoever the vendor is, make sure you get an On-Site (call-out) warranty in with the deal!