Just remember: Don't buy Black BB's from the get go.. It makes it VERY hard to set your hopup right

(Not to mention, people will think your mags are empty)
And I second the fact that the difference between .20 and .25 is astounding. After a while, .20's just go ass-wild and spin every which way (with and without hopup), whereas I find .25's a nice decent long range round..
If you're using something short ranged like a MP5K or a pistol, then by all means: Buy in bulk with .20's.. But if you're planning to hit something at distance; stick with .25's.