Links...
Linksys
Microcenter
The 500-pound gorilla of networking. If you need networking hardware, these are the guys that wrote the standards. They aren't the cheapest, but they are best-in-class and state-of-the-art.
Microcenter is a place to find that unusual hardware piece that no one else seems to stock. A huge selection of almost every brand imaginable. I like to order online and use their 18-minute pick up guarantee.
Best Buy
G.Skill
Best Buy is a great place to buy hardware and software for you computer. Their price-match guarantee makes choosing a retailer much easier.
G.Skill memory products (DRAM) offer the best performance and very reasonable prices.
CPU-ID
CPU-ID (and it's cousin, HWMonitor) are two great (and free) tools to find out what is going on with your PC. It will tell you how much memory you have, and about your motherboard, graphics card, etc.