Don't worry about moving from xp to win 8..the operating system is similar enough you won't have any problems..as far as buying a new pc..look to buy from newegg.com or tigerdirect.com for the best deals.
Alternatively..use a local guy who builds PC's and tell them what your budget is then compare all the specs between what the builder can do versus what you could buy for the same price.
_________________________
You know something bad is going to happen when you hear..."Hey, hold my beer and watch this"