I’ll give a very brief answer, enough to get you started…
“Best brands” is a misnomer to me. If you get a Dell, HP or something, you may be stuck with certain proprietary replacements, only through the manufacturers directly. After getting a better deal by having one built, that’s what I recommend you do at least. Research a local shop that is at least is recommended by the local BBB, and check around to see if your friends or people you know have frequented there, to get an idea by word-of-mouth (usually pretty accurate I find for such a thing).
You would be better served by using an AMD CPU, for the fact it still access the RAM faster than Intel counterparts. Also, avoid “genuine Intel motherboards” if you have a system built, as they sometimes have a nasty shortcoming of their USB ports shorting out when you insert a flash drive (we had several at work go where the USB ports shorted out for apparently no reason like that).
The best brand of motherboard you could get is Tyan, although you’ll pay about 50-$75 more for a Tyan mobo than a similarly featured Gigabyte one. In my research, your best motherboard choices in best > very good > good would be Tyan > Gigabyte > MSI (the tech guy at work added the MSI). A critical feature on new mobos is to get one with as few “X” capacitors on it as possible, with the majority being “K” “Y” or “V” designs. Many Dell motherboards (at least ones that aren’t made within the last 2 years) were notorious for having too many X capacitors (these are the cheapest ones, that are most likely to go out), and invariably they bulge, go out or even pop and go out. Anyway, a good mobo of the Gigabyte variety with the majority of the ‘good’ capacitors will run you 90-$100, ordered from a place like www.newegg.com
On AMD X2, that’s a dual core processor
Get a good case with at least one intake fan and one exhaust. Some of the best cases are made by Cooler Master (that won’t break the bank), that also have really good airflow. You can probably score a good Cooler Master Centurion case from newegg for $50 or less on sale (that already has 2 fans, one intake in front and on exhaust at back).
Don’t go cheap on the power supply (PSU). I recommend an Antec, or if you can find an equivalent power supply that has an 85% efficiency rating or better. I would recommend somewhere in the range (minimum) of a 430 watt Antec PSU, as these could be found on sale at Staples/Circuit City or elsewhere for ~$40 or so, with some in the 500watt range also for that price (or to ~$45) or a little more, depending on the sale.
Unless you need some super DDR-Ram, you could go with Kingston Value RAM for whatever Mhz matches what the mobo would take. A recent sale at Circuit City on (IIRC) 533 Mhz DDR-RAM 1 Gb was $39.99. I would have at least 1 GB DDR-RAM in the computer.
A little more than I thought I’d put up, but I went with the flow. This will give you some more ideas.