Depends what you want,do you want mods that will make your car faster in a straight line or mods that will improve the dynamics of your car and make your car faster from a to b?.
If you want cheap bhp ,look at induction and de cat ,if you want a quicker a to b car look at a set of road legal slicks ,tyres are the most important component of a car and will give you more gain than engine ,suspension mods etc ,no point having lots of power or trick suspension on standard tyres as you wont be able to extract all the potential,any car can only corner as fast as the grip its tyres give,increase corner speed and you increase overall speed dramatically.
Dave.

Spot on Dave, to have the power you need to be able to carry it. In this instance it is purely bottom end budget and releasing some restricted performance. Then it becomes an obsession! :-D

I have cheap budget tyres on the back of mine at the moment and they are really crap not only do they weigh half a ton they have no grip either.

What ones do you have out of interest?

I use Hankook K104's and they are excellent tyres in both wet and dry, superb on track too and not expensive circa £70 a corner.