How much hardware to buy?

Folks: It's way past time for faster horses, and I was hoping for some advice about horsepower. We have an RS/6000 model 390 (single processor) box w/ a 1G byte Informix 5.02 database and 256M RAM. Question is, when we migrate the same database to Informix 7, how much more horsepower do we need? We are looking at an RS/6000 model R50 w/ 4 processors and 512M RAM. Is this overkill? Is this not enough? We use r4gl quite a bit, and also lots of ad-hoc queries through I-Star during the day. Heavy batch processing happens during the night time. Is anyone running a similar configuration? What gotchas popped up during the conversion? Thanks in advance, Mike Lester USAA Investment Management Company