Cpus to stay away from
Brand A (AMD)
Semipron (1 core)
Athlon with no number (such as 2650e) (1 core)
Brand B (Intel)
Anything Atom (either the 1 core or 2 core variety)
Anything Celeron (either the 1 core or 2 core variety)

You want one of these for the Cpus The last ones are the faster cpus

Brand A (AMD)
Good) Athlon II x2 (2 cores)
Beter) Athlon II x3 (3 cores)
Faster) Athlon II x4 (4 cores)

Brand B (Intel)
Good)Pentium Dual Core (2 cores)
Better) Intel Core 2 Duo (2 cores)
Faster) Intel I3 (2 cores, but it has a trick called hyperthreading to trick the computer into thinking it has 4 cores)
Faster) Core 2 Quad (4 cores)

The Athlon II x4, I3, and Core 2 Quad are all about the same speed. Some things one of the 3 cpus are faster in, some things the other cpu is slower.

Athlon II x2, Pentium Dual Core, and Intel Core 2 Duo are technically all you really need but the other cpus are faster. You will be satisfied with the speed of the 2 cores computers (and most people consider these cpus very responsive), but 4 cores are only like 100 dollars more on average. (Cores is how many cpus, how many engines the computer has)
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You want 4 GB of memory (RAM), and no more than 6 GB. You will not notice the difference between 6 and 8, and probably never really notice the difference between 4 and 8. While 8 is great you are wasting money getting it. 4 to 6 is the sweet spot.
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Don't worry about the harddrive. You won't fill it up. The hard drive is storage.
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The CPU is the speed of your computer.
The Memory is how many things you can do at once.

So imagine what you do on the computer, the data, are little cars on the highway. If you are doing more things at once then there are more cars on the highway.

The CPU is the speed limit of the highway
The Memory is how many lanes the highway has.

A faster CPU means a faster speed limit, you always benefit from a faster CPU. You only benefit from more memory if you are doing a lot of things at once, things that would cause a traffic jam. If you have a traffic jam, then it doesn't matter what the speed limit is, your highway is clogged and feels bogged down.

When you are not using the data, instead of it being on the highway, the cars are stored in the parking lot, they are stored in the hard drive. Hard drives are big enough now a days that people rarely fill them up unless you store hours upon hours of video on your computer.

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Here is some rundown of computers that Best Buy offers

Good (Good CPU, 4 GB of Memory) $399
http://www.bestbuy.com/site/eMachines+-+Desktop+with+Intel%26%23174%3B+Pentium%26%23174%3B+Processor/9703523.p?id=1218153427158&skuId=9703523

Same Tower as above but bundled with 18.5" monitor $469
http://www.bestbuy.com/site/eMachines+ET1831-07+Desktop+with+Intel%26%23174%3B+Pentium%26%23174%3B+Processor+%26+18.5%22+LCD+Monitor/9999126600050004.p?id=pcmprd126600050004&skuId=9999126600050004

Good (Good CPU, 6 GB of Memory) $449 This is pretty much the same as above but more memory
http://www.bestbuy.com/site/Gateway+-+Desktop+with+Intel%26%23174%3B+Pentium%26%23174%3B+Processor/9605222.p?id=1218130487022&skuId=9605222

Best (Best CPU, 6 GB of memory)+20" Monitor $699
http://www.bestbuy.com/site/HP+p6310y+Desktop+with+AMD+Athlon%26%238482%3B+II+Processor+%26+20%22+LCD+Monitor/9999126600050002.p?id=pcmprd126600050002&skuId=9999126600050002

Best The gateway you posted. Now the Gateway has a HDMI port so you can connect the tower to your TV and use your TV as a monitor as long as your TV has a HDMI port.

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Danu where do you live again? What other stores do you have besides Best Buy?