pixel mesurement

a while ago i bought a 2d book on directX, i found all of the initialization very confusing and it seemed like i had no control over what my program did, so following a friends advice i went to opengl, i am now programming a 2d breakout clone with opengl, ive gone back to that book as a source for 2d programming theory and implementaion, anyway, in that book they measure where sprites are and movement in pixels, opengl uses floats( 1.0f etc) which is a faily large unit(in 2d) does anyone know how i can set where a player is on the screen using pixels? like (320, 271) would be approx in the middle of a 2d 640X480 screen, any help would be appreciated, and if its all pointless and better to use opengl''s floats, plz just let me know, thanks ,Matt -= kill one your a murderer, kill thousands your a conquerer =-