Good idea. a tower is, for many users, be better investment than other types of PC systems. Like a laptop.
You say tower, I think you mean a full desktop computer ready to go. In other words, a full-size desktop system. Do you already have a monitor?
Unless you are a avid gamer, a low cost tower system will do what you need. A consideration is if you comes with full software installed. Some models now have MS Office as a complete package, not a trial.
There are now several big companies in the desktop system market. Some vendors have a reputation for having lower prices. Be careful. Check the specs and return policies.
Asus has a good reputation. Sony? no comment. HP is current the big seller. Apple is very big. But Apple tends to be pricy, yet a very good choice.
Dell Inspiron 570 Desktop is an entry level desktop with very good reviews overall. About $300, but no monitor.
CyberPower Gamer Xtreme 3000 About $1200 . Yeah, its a mean gamer.
Just about everything else is now either a laptop or a All-In-One. Sorry, I can't recommend the All-in-one.
HP also makes a entry level system. I just didn't find it in a Google search.
If you live inside the USA or its territories, check bout WallMart Locally; I saw a full tower system with a 20 inch monitor and Office install for about $500. It was a major brand, just don't remember which.
In some areas if is better to by from a store withing a reasonable distance from your home. You get to see the actual system before you buy it. And return is much easier. Some retailers will let you return or exchange withing 10 days with not penalty.
Here is an interesting ite:
1. Apple (3). . . . . . . . . . 108.9 Million . . . . .17%
2. Nokia (1) . . . . . . . . . . 100.3 Million . . . . .16%
3. HP (2) . . . . . . . . . . . 67.5 Million . . . . . 10%
4. RIM (6) . . . . . . . . . . . 47.0 Million . . . . . 7%
5. Acer (4) . . . . . . . . . . . 43.8 Million . . . . . 7%
6. Dell (5) . . . . . . . . . . . 42.7 Million . . . . . 7%
7. Lenovo ( . . . . . . . . . 36.2 Million . . . . . 6%
8 . Samsung (-) . . . . . . . . 26.0 Million . . . . . 4%
9. HTC (-) . . . . . . . . . . . 25.0 Million . . . . . 4%
10. Toshiba (7) . . . . . . . . 22.1 Million . . . . . 3%
http://www.brightsideofnews.com/news/2011/2/24/top-10-worlds-biggest-computer-makers-revealed-surprised.aspx
The abve is about market share and recent changes. But just because a company is doing a great business does not always mean they have the best product. Dell has dropped a bit. Still, my personal preference is Dell. But I might change my mind.