i looked at a northstar engine in a friends caddy the other day. the plugs appear to be mounted in the center of the valve covers like the hemi's is this a hemi head engine? the car was very peppy 300hp rating.

No it's not a hemi, it's a 4 valve per cylinder head like the FoRd Cobra, Honda 4 banger and the discontinued Corvette ZR1 LT5 engine. I guess you would call it a double wedge head.

Four-valve pent-roof design! Currently one of the best designs around, without going to four+ valves.

Is this is same mill in the Olds Aurora?

The Caddy is a 4.6L engine. The Aurora has a 4.0L V8 that is based on the Northstar Caddy. Some call the Aurora V8 a mini-Northstar.