They were only made in one year (1974) in the USA, so parts are hard to find. Hopefully yours is in good shape. In the UK, the 260Z was made a bit longer and was actually a nice car because it didn't have all the emission stuff the US version had.
1) V8 conversions - $3,000 minimum
2) 2.8L conversion with 5speed - crate motor, SUs, and 280Z/ZX 5speed will run you about $2,000 minimum
3) 2.8L Turbo from a ZXT with 5 speed - $2,000 if can get a nice turbo motor with all the FI and ECU stuff (personally I wouldn't go this route, but many have and like it)
The V8 conversion is not that difficult nor expensive and would give you the maximum bang for the buck. The 2.8L conversion will keep the handling characteristics of the car and is a straight forward conversion.
Remember that when swapping the 4speed for a 5speed, if you don't swap the rear differential, you'll have a doggy car.
All-in-all it's always cheaper to buy someone elses conversion than doing it yourself. I saw a 240Z with a 5sp, 2.8L motor, E31 head, headers, tripple carbs, and Toyota 4x4 brakes selling for around $5K. Oh yeah, the body was rust free and straight. Try and do that yourself for under $8K.