The default speeds for a GF4 Ti4600 are 300 core and 650 memory. Considering your core OC is much higher and your memory is only 4mhz lower, then yeah, I'd say that's a good overclock.
Does your Ti4400 have heatsinks on the memory? You could probably go a little higher with the memory if you had heatsinks or more aggressive cooling aimed directly at the card.
Otherwise, you've basically got a 4600 right there.....be happy.
