Motu 324pci Vs. 424pci Vs. Firwire., stutering and latency Options

[hani]

im currently using :

mac g4 1.3 ghz osx 10.3.9
motu pci 324 w/ 1224 IO.
cubase 2.2sx.

i have set my buffer very high (1024 samples). and still getting stutering .
and cant play any vstI synths in real time due to very bad latency .

i cant set the buffer any lower because the stutering will increase .

should i switch to Motu pci 424 or firewire? or switch brands .. will the latency & stutering issues improve?

any cubase users out ther? what do you reccomend? what are you using?
i dont need much!! just the basic 8 out/in digital I/O and if midi is included that would be cool too.

please help.