Your current lens collection is quite nice, and will serve you well for what you want to do, whether you stick with an APS-C body or go 'Full Frame' (so long as you stick with the A-Mount.) The A700 would be an upgrade from your A300, as would the A500, A550, the A700's eventual replacement, and of course, any of Sony's full frame bodies.
I will also mention that Sony's 24MP 'Full Frame' dSLRs are the new favorites among fashion, portrait, and wedding photographers, especially when paired with the autofocus Zeiss lenses that are only available for the A-Mount. If you were shooting sports/action/wildlife, I'd suggest that you look elsewhere, but for pet photography, you're already where you should be.
- The lens is the thing.
- 'Full Frame' is the new 'Medium Format'.
- "One good test is worth a thousand expert opinions." - Tex Johnston, Boeing 707 test pilot.