Forget looking for something that will last you ten years. That's bad criteria, instead find the things you do now regularly, the things you do once in a while and the things that really take a long time for your computer to do now.
A few things you should keep in mind is your obvious Internet usage. This message board stuff doesn't take much horse power to do, but the movies and pictures might put some demand on your new system. You will eventually upgrade to broadband so that's a key issue to take into account.
For the rest, it's up to you. There is no limit to what you can pay so forget that aspect. You don't need to be the fastest but you do need to keep up. If that's the case, look for something name brand with the features you want.
Custom made is really the way to go but unless you know what you want, you're just going to make a mess of it.
Now is the time to look at the new stuff you can do with a computer to see if that's what you want to do as well. Photos and home movies, music and art are all at your fingertips now.
One last bit of advice. Along with your computer get a second external hard drive to back up on to and use for storage of things like pictures and music. So if your main drive fails, which it probably will at some point, you won't lose stuff. Back up early and often.