If all you want to do is record riffs for guitartricks though, you can get by with a lot less. On a Mac all you need are the rca inputs or built in mic on older models and use a freeware app to record.
You can set up a really cheap system with a multieffects box for your guitar, plugged into a cheap 4 channel RadioShack mixer and straight into the rca inputs of the computer. (or forego the mixer altogether and just use a 1/4" to rca adapter to go direct) If you want to go all out you could use a shareware midi player to drive a cheap GM sound module and mix that onto the other 2 channels. All you'd need is the mixer, midi converter, sound module and a couple of cables and you've got a half decent setup without needing to spend hundreds on software and the latest mega fast PPC.