I would wait until Macworld in January to buy anything mac-related.
The Mac mini which you refer to has not been refreshed in quite a while hence why its specs are so far out of date for the money. Chances are it will be renewed really soon.
You could also get an iMac which are pretty good and you get decent value for your money.
The thing is, too many people focus on technical specs and not the usability. In reality, for most users who use the internet, chat online, watch videos, edit pictures, etc. a computer that is 2.0Ghz will be just as good as one that is 2.4Ghz or more -- that means it will do what you want it to do, and it will do it pretty quickly.
Specs matter most only in benchmarks where their importance is magnified by the use of bar graphs. In reality, whether it takes you 3.5 seconds or 3.7 seconds to encode a song or 2.6 seconds versus 3.1 seconds to launch a program, you could care less and probably would not notice the difference.
With a mac, the OS is also faster and more efficient than Windows and the software packages like iLife are far superior to anything you can get on a PC for money -- and they are INCLUDED in all macs.
So don't get too hung up on comparing technical specs directly with PCs to see which is the "better value"
You get a lot for your money. First, you get iLife with all macs, which is a $79 or $99 program separately. This suite allows you to edit movies, organize and print photos, create web pages, etc. very easily and with many good options. It will unleash your creative side even if you never though you had one before.
Also, with macs you don't need to worry about viruses or buying virus protection software. That saves you $40 or so every year you own your mac, plus you get priceless peace of mind and you never lose your stuff because of an infection.
Since Apple controls its hardware carefully, you are far less likely to have any major stability issues or crashes. Since the OS is pretty secure and well developed, it never really "slows down over time" like Windows does, saving you the hassle of having reformat regularly.
Everything just runs smoothly so you spend more time enjoying your computer rather than maintaining your computer.
And of course if you get an iMac or something it will lessen the clutter on your desk. That was one of my favorite things about it -- and you won't have that if you build a PC and slap Mac OS on it.
With a mac, you can finally think of your computer as a tool that you can use to do cool things with like create movies, edit photos, surf the web, get work done, etc. rather than as a machine that constantly requires your attention just to run properly.
And with a genuine Apple computer you also get a nice looking item that will go well with your room.