Use these two sites and you are good...
www.tomshardware.com - research
www.newegg.com - purchase
People asking for computer assistance need to start off by stating their budget since that is the largest obstacle. Second are you looking for a laptop or desktop. If your just looking to spruce up your current system it really comes down to seening what the largest processor it will support(pay attention to the cache), adding more memory, and one item people always exclude is a better performing hard drive.
People think of a hard drive as just storage, but like anything there are low spec drives and high spec drives. Most likely you either have a 5400 or 7200rpm drive w/ 2mb cache. If you have a 5400rpm drive that is a huge bottleneck, you need to upgrade now and will be blown away by the performance upgrade(looking at all you laptop and low end desktops). If you have a tight budget I would upgrade to a 7200rpm drive w/ 8mb cache but if you got a decent budget I would get a 10k rpm 8mb cache drive and if your motherboard supports it go SATA vs the typical IDE.
The next large bottleneck is under maintained operating systems. People can yap all they want about MS and yes 95\ME\98 sucked but XP\2k\2k3\Vista are all pretty strong operating systems and will run well if you maintain them. Dont run under the admin account, dont downloard porn from a site that is free(free porn means free access to do whatever they want to your system), run spyware software everynight at like 2am(dont forget to set it up to receive updates), always get MS security update(release 2nd tuesday of every month), make sure your antivirus is updating and dont accept SPAM!!! etc etc etc
Its kinda like riding a motorcycle in sandles, shorts, no shirt, no helmet and never maintaining your motorcycle... you have that choice but its just not a wise one.
Most decent size cities will have computer stores that do free or cheap diags of systems so dont be scared to build your own.
Feel free to IM if you would like my assistance.. about the only way I can give back to this board since my motorcycle knowledge is still pretty small.