To answer your question the voltage of the pack will not affect the amount of run time with LIPO batteries. What will affect the runtime is the mah rating of the pack, this is the measure of the packs capacity.
For a wraith I would say something between 4000mah and 5000mah would do the trick. Remember the larger mah batteries are bigger and that means that they are heavier so it may be better to have two smaller 3000mah or 2500mah packs rather than one large 5000mah pack.
Now for voltage you will have to make sure that your ESC has a lipo cutoff in order to run LIPOs safely. Also if you want to run 3s (you should it is great) then you will need to make sure your ESC will handle the extra voltage.
A great resource for cheap LIPOs and chagers is
Shop for RC Hobby Parts and Upgrades - Digital Servos, Brushless Motors, ESC, LiPo Batteries/Chargers for RC Helicopters, RC Planes, RC Cars, Robots. You will want a battery with a 25+ "C" rating.