You might be better off checking into the cheaper Dell PC's that are available. I believe they are running around $400-$500 so this would probably be much cheaper than trying to make any upgrade on the existing PC that you have.
Do you feel like you can do the work yourself? If not then you could be looking at $40-$50 per hour to do the work for you. It is a little hard to estimate how much time would be needed to do the work but you would need to take this into account.