I agree with Sparker that your power supply is to small and you're going to have a rough time fitting that 5770 in the case. (The Mini P180 is what my current rig is built in.) You can get the 5770 in the case. But in the process you'll have to take the drive bay out that is directly behind the video card. You'll loose the ability to add 3 drives when you take that out. You'll also want to add 1 Antec 120mm transparent 3-speed case fan to the front lower fan mount and let the motherboard control that as the system fan. Set the other 2 fans on medium. The switches are on the back.
As Sparker stated, I wouldn't look at anything lower than a 500 watt power supply. The Antec Earthwatts EA500 for $69.99 plus shipping is a good choice. According to the HIS specs, the EA500 will give you 50 watts of "head room".
As for the CPU, I run the AMD Phenom II X3 720. This is going to run you about $17.00 more than the 550. I'm very satisfied with it's performance. It can be overclocked just like the 550. But if you plan on doing that, make sure you have a good after market CPU cooler. Like the Sunbeam CR-CCTF 120 mm Core-Contact Freezer CPU Cooler for $34.99 with free shipping.
Mark NØLXX
First computer built - 286-16 MHz. running DOS 3 and Windows 2.0 BETA
BBS SysOp for 6+ years (2 lines, 1 computer)
Antec Mini P180 + OCZ StealthXStream 500 watt
GIGABYTE MA78GM-S2H + AMD Phenom II 720 BE OCed to 3.4 + 8GB G.Skill PI Black DDR2 800 (CAS 4) +
Sapphire Radeon HD 4830 1GB + Windows 7 Home Premium 64-BIT