i have a 20 inch imac and it's a pretty good computer for a consumer level beginner or advanced beginner, but it's not a professional machine like any G5 power mac, expandable and able to handle ram to 4-16 gigs, card expansions, and processor expansions (thus doubling the machine's life)
the drawback of the power mac is that you don't have the screen included...but in the long run, you will be so much happier you bought a professional machine...i have two power macs, had one before that, and have the imac and ibook
an ibook, mini mac, or imac are not bad, but you are not going to get the performance, expandability, and long useable life as a power mac...the power mac will last 2 or 3 more years than my intel imac, which is very limited on ram expandability and is still only a 32 bit processor, and the extra years will be worth buying the tower and a nice apple monitor
today's intel imac may beat the oldest g5 power mac, but you cannot expand any imac so it will only take 12 months or so to feel its age and then you will be stuck with one processor configuration, one sound card, one video card, and a very limited amount of maximum system ram
if you can, buy a computer for the long term, especially if you are a student and you want the machine to last you several years of college and possibly graduate school or first few years of the working world