You'll probably want to check out KVRAudio, they have a good forum and you can find some AMAZING free plugins on there, everything from synthesizers to effects. Great website.
If you want an in-the-box solution you might want to look into buying logic (assuming you're on a mac), it has pretty much everything you'd need included in the program, and its well suited to recording guitars. You'll also need an audio interface to get the sound from your guitar into your computer. I have an M-Audio Fast Track Pro, its a great little interface that'll last you for a long time.
Anything you can do with software though you can also do with hardware. I'd almost say you're better off just getting some effects modules and putting together cool signal chains. I've done a lot of sound design with software, but its honestly way more fun with hardware. Tactile control over a mouse and screen any day.