If it is a top mounted case the psu fan faces the inside of the case. No problem as long as you have s good exhaust fan.

OK if the seasonic psu if turned up side for top mount case then the power switch would facing the outside of case and the fan would be facing downwards toward the mobo and cpu fan and cooler

The HD4650, according to the table in Wikipedia, is only 48W TDP, 68W for the Pentium - could probably use one of the FSP 300W units (ebay) that tend to have very strong 12V capability - I got a FSP300-60GHN a while back, there are a couple of similar models that may be different, but that one could deliver 288W on the 12V - something which even some cheap "500W" can't do!

The "big fan" is the modern style, and it would usually face down - in the bottom of a "PSU LOW" case, there is a filtered intake, so the PSU fan can be slower and quieter as it breathes from outside

To answer the original question, how do you tell a system that needs lots of 5V power from a system that needs lots of 12V power? Simple. Does your motherboard have this:

See that extra 8-pin CPU Power socket on the bottom right? That's an extra 12V power delivery rail specifically for the CPU. If it has even the 4-pin version of that, then yeah, the system draws heavily from the 12V rail. That includes everything Pentium 4 and later on Intel's side and just about everything that came after the AMD Athlon XP line (although there ARE a few late-term motherboards for Socket 462 [Socket A] that were designed to use the 12V rail to power the CPU instead of the 5V rail and come with a 4-pin CPU Power Header like this, but they're very rare!).

ok my mobo i am using for this XP build has the 4 pin version

the mobo i will be using for my win98se build does not have anything like that

got the seasonic 550w psu for the XP build and so far everything seems fine and fit in top mounted case no issues