Well there could be a large difference in performance depending on which GPU Sony go with, all that's known is it will use an AMD southern islands series card but that covers quite a range.
Regardless of which one it uses it will be more powerful that the new Xbox's 6670, just by how much we don't know yet.
6670 vs 7750 (low end southern island)
Radeon HD 6670 (OEM) vs Radeon HD 7750 – Performance Comparison Benchmarks @ Hardware Compare
6670 vs 7970 (high end southern island)
Radeon HD 6670 (OEM) vs Radeon HD 7970 – Performance Comparison Benchmarks @ Hardware Compare
What's interesting about this new Playstation console is the name Orbis. Orbis Vitae is latin for "circle of life" which sort of indicates they may be going for increased functionality with the PS Vita.
It's all just speculation at this point anyway, hopefully we'll see some solid specs at E3.