My 650w Corsair PSU has a fan ontop of it, but sometimes I feel the power supply gets warm, I feel the heat from outside my Case.
Is this normal?

Right now i've got a 140mm fan sitting directly ontop of the PSU (it's upside down btw, psu on bottom of case) and it's blowing directly into the psu.
is this safe?

We need to know your setup. What components you have, etc. Until then we can't
determine if your PSU is near overload.

Blowing cool air onto the psu won't do any harm. Making sure you have enough vent
at the exhaust fan at the rear, and also plenty of room surrounding the back of the case.
Typically, you'd feel quite a bit of heat from the PSU especially if you're overclocking.
Remember this is the power unit of the system. 650w is a lot of heat.

I'm running
1x 8800GTS 512mb
ECS nforce 4ma Motherboard
2GB 667 DDR2 RAM
AMD x2 3800+ 2.0ghz w/ Arctic Freezer 64 Pro Heatsink + Fan
Antec 900