It really depends on how dire the computer situation is. If you need a new computer then you should definitely get a new computer. However, if you're satisfied with your computer at the moment, then I'd say go for the guitar. I would also recommend selling your old guitar, but I completely understand the packrat thing. I'm the exact same way. I've got two decent guitars and 2 1/2 total junk guitars. I could sell a couple to get a really nice one, but I just can't justify spending $1000+ on a guitar. which doesn't make sense seeing as I can justify buying 2 $400+ guitars.
Ok. That got totally off topic. To summarize, buy the guitar unless you really need a new computer. You seem like you take your music very seriously.