You don't really have too many choises right now, either exchange the PSU for a smaller sized one or get a new case.
Since you have an older case, I'm guessing you aren't into modding and light shows (like me
). There are many low priced cases on the market that will fit the PSU for under $50 if you're not picky. I personnaly own an Asus Vento case which costed me like $30, it's just fine! I have a Coolermaster Silent Pro 500W modular PSU and 2 x 120mm fans installed in it and it's silent. I would go with a new case.