|"p6777c - Athlon II X4 635 2.9 GHz, FSB up to 1333 MHz|
p6631p - Phenom II X4 830 2.8 GHz, FSB 5200 MT/s"
"This is really a MHz vs. MT/s question"
That's a tough one, especially since AMD hasn't used a FSB in their systems since 2003. So either you copied that stuff wrong, or whoever wrote the description for HP is clueless about computer hardware. The replacement bus AMD uses is called HyperTransport. There have been several versions so far, starting with v1.0 at 800MHz (aka 1600MHz or 1600MT/s) & currently at v3.0 at 2600MHz (aka 5200MHz or 5200MT/s). The HT bus is bi-directional causing it's speed to be advertised in different ways, similar to the way you see DDR/DDR2/DDR3 RAM speeds advertised in different ways. If that's too confusing for you, just look for a system with a motherboard that supports HT 3.0.
"How do I compare 2 PC's ??"
You need to look at the entire picture - CPU, motherboard, memory, graphics, HDD, power supply, etc. But I can tell you this, the Phenom X4 is better than the Athlon X4. The 100MHz difference in clock speed means next to nothing. Other than that, you gave us little to go on & I'm not digging up the specs.