Critique my build... is an H55 mobo ok for gaming?

Your list of parts looks appropriate and good.

The difference in CPU will not be noticeable unless your games are multi core enabled and cpu intensive such as FSX. Otherwise, your games will be limited by the power of your graphics card. The GTX250 will give you decent gameplay at 1920 x 1200. A stronger card for that resolution might be the 5850.

The reason H55 motherboards are recommended is that they will usually have two pci-e x8 slots.
That is of value if you plan on using two graphics cards. If you want to use two high end cards the X58 with multiple x16 slots is better. Crazy.
Just use one good card if you can. Multi gpu systems may have scaling issues with some games, and you have to pay more for motherboards and psu's

The H55 chip has another advantage which you might want to consider. It supports the clarkdale cpu's with integrated graphics. They are very strong 2 core cpu's with hyperthreading that might do the job even better than the 750. At the same price point(about $200), you can get the i5-660 or 661. It's stock clock rate is 3.33 with turbo up to 3.6. Do not plan on using the integrated graphics for your gaming monitor. It will be adequate for civilization, but not fast action games. You can use the integrated graphics vor a static secondary monitor to good effect.

Get a case that you like. The mini P180 is a good one, and very quiet. If you find one with free shipping you can save about $20.

---good luck---