If you buy a built system, you will still need to learn about all of the specs of the system. A lot of these systems have an integrated graphics and sound card. An integrated video card does work, but it cuts down on the available system RAM, and will not usually work as well as a dedicated video card. If you have 512MB of system RAM, around 128MB of that RAM is reserved for video processing, so your effective system RAM is only 374MB. You can get an upgrade and get a dedicated video card, which is well worth it. Make sure to look at all of the specs for a built system, not just the processor speed. Look at the CPU maker, the motherboard manufacturer, amount of RAM and the hard disk specs. The best thing about a complete system is the warranty package and support. Ask a few people who they have bought from, and how they liked it.
Building a system requires you to do a bit more research, but is well worth it in the end. You can pick the best components and manufacturers, and get a system that will run slightly better to amazingly better than a system you buy. You can also end up building a lemon, and spend a lot of time correcting it. Make sure to buy your components at a store that has a good return policy, and don't lose the recieipts. A personal story: I was upgrading to my new system, and wanted it to be a screamer. Went out and bought all of the components and 512MB of RAM, and set it up. The system kept resetting after 15 minutes or so. Took back a lot of components, and eventually found out that the RAM I bought was not working properly. It was supposed to be PC2700, but wouldn't work in the system unless I set the bios for PC2100 RAM. It took me a while to figure out how to correct it, and the return period had lapsed, so I had a system that should have been 1.6Ghz, but was instead running at 1.15GHZ due to a RAM problem. It worked fine underclocked, and eventually I replaced the RAM and it ran fine at 1.6 GHZ. Building a system will teach you a lot more about computers, and will help you to solve problems when they occur later down the line.