500 watts is a popular segment these days. It's more than enough for powering an Ivy Bridge or Haswell CPU and a single mid-range or high-end graphics card. There's a lot of competition in the pricing in this segment. Spending money on a decent PSU is not a small part of the total price of building a capable PC system. For this reason manufacturers try to keep prices for their 500W PSUs as low as possible.
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips