As you stated you run XP and that suggests you *MAY* have an EIDE type drive, so verify. Otherwise, you have a SATA type, just google to check what the connections will look like. Of course, the decal/label on the HD itself will ID what it is and go from there.
As for 320gb( you can get a larger HD of same physical size but larger capacity and typical would be 500gb or even larger up to 2gb if that makes you happy. Under XP OS, 2gb is the best IMHO you'll ever want. as for refurb, sure you can go that route and in most cases you'll do fine, typical warranty is 90 days, so be prepared if the need arise to send back. However, brand new isn't that much of a difference and warranty is usually 1yr.-3yr., read the fine print. good luck
tada -----Willy 