It depends on how much money you have to spend. First, look at the heads and intake. At around $1000 a pair, the GT40P's have good flow for a good price. For a few more kw, look at some alloys, but add a few $1000 to the price. To go with the heads, get a GT40/edelbrock/trickflow intake because the stock intake is the most restrictive part of the engine. Then look at a cam and roller rockers. The crane 2031 is a very good cam for mid range torque and high rpm improvement, and is EEC friendly (there are lots of good cams too). You will also need a larger throttle body, MAF and injectors to cope with the extra power.
BTW, porting your stock heads will probably give you the same flow as the GT40P's. If you go for the GT40P's you will need new 90 degree sparkplug leads to fit. Also, GT40P spark plugs are expensive at around $20 each.