Here's my input on this since I take technology seriously. I know it's TL;DR, but if you read it, you will not be disappointed.
1) Brand wise I would suggest Toshiba or HP. I've been with Toshiba for years and love their work.
2a) There's three things you should look for. CPU, RAM, and OS. The CPU gives you the brainpower, while the RAM gives you the muscle. I3 processors are the basic standard for commercial use. They get the job done but can only go so far. I5 is the next level up. Their power is way better than I3, but the price for them is far more expensive. I7 is the best of the best, but way too expensive. If you can get an I5, GRAB IT!!! You will not regret it. Now whether you want Intel, AMD, or some other processor is an entirely different subject.
2b) RAM is just as important as the CPU. Most standard desktops and laptops give you 4gbs. It's good and can do a lot, but you can feel the limitation if you run a lot of programs. I suggest looking at your computer's specs to see how much RAM you can install. You don't want to buy 16gb of RAM and you computer can only handle 8gb. It's wise to see what PIN size your RAM chips are. 204? 214? Knowing the right PIN count will save you money and make sure your products will work. Brand wise, I give all praise to Kingston and Corsair. Both make good quality products. If you want to max out your RAM, buy dual-channel sets. (i.e. your computer can handle 16bg of RAM. Buy a 16gb(2x8gb) RAM chipset. It may be two 8gb RAM sticks, but they're specifically designed to run at full capacity when they're used together. Two separate, meaning not dual-channel sets, do not run at full capacity.) Personally I'm after the Kingston FuryX 8gb(2x4gb).
2c) Your OS is important if you want it to be. A 64bit system is better than a 32bit because you have more room to do more things. Besides that, the 64bit can use RAM to its fullest. (if you have a 32bit system with 4gb of RAM, you're really only getting 3.4gb of RAM. this is because there isn't enough brainpower to use it.) A 64bit system isn't always more expensive than a 32bit. It depends on what you're looking at.
3) If you just want to switch out parts, Newegg and Tigerdirect are the best that I've found and used. Newegg gives you amazing deals, but a medium range of products. Tigerdirect gives you a massive selection, but prices can make you bleed.
4) If you're going to buy a new computer new and not upgrade, it is HIGHLY urged you update everything! This means Drivers, Audio/Video/Graphics cards, everything. Use this link:
Give the short video a watch if you want to know everything.
5) Gaming computers are king! This is just my input, but they're built to handle tons of jobs at once. :D