My father use to do tax returns part time... He was an engineer in the day time, and during tax season he did taxes..
Anyways I suggest you buy any good games you don't have... If I had $500 to spend, I would probably buy new headphones.
Congrats on your refund!
Some games that I highly reccomend are DKC Returns for your Wii, Yakuza 3 (which should be pretty cheap now with 4 coming out in March), Gran Turismo 5, or any other games you missed out on recently.
Maybe even a PSN card for like $50 and just add it to your account, so in the future you don't have to worry.