wow. no offence but you really have no clue. you have 256mb ram and are planning to move to a B&W G3 upgraded to a G4 1ghz?
first off the mini has a 167mhz bus, vs the 66mhz bus your B&W will have with a 1GHz G4. those sonnet G4 1ghz have to underclock the B&W bus from 100mhz down to 66 so that they can run stable. no way to change this either. so its 1ghz on 66mhz bus vs. 1.25ghz on a 167mhz bus. do the math. plus the ram will operate MUCH MUCH faster on the mini. a B&W with a G4 1ghz gets a memory score on xbench of about 30-35 vs. a mini that gets up to 145. here is a B&W w/G41ghz vs. a mini 1.25:
http://ladd.dyndns.org/xbench/merge.xhtml?doc2=111594
some tests are many times faster.
for MUCH less money you could keep the mini and add a 7200rpm hd and up the ram to 1gb. that config will be MUCH faster than the upgraded B&W. i'm so confident of this that I swear my life on the fact.
wake up to the real world and stop thinking in a way thats going to cost you more $ and leave you with a slower system.