Some 4x media will burn in the 2500A at 8x, but not all. My 4x Ritek discs burn at 4x just fine, on par with what I expected. 8x media is hard to come by and expensive; I'd save your money and stick with 4x.
As for disc type, I'm using DVD-R discs. Haven't used +R's, so I can't tell you if there's a real benefit. I got the -R's because they were a bit cheaper, and rumored to be slightly more compatible on older DVD players.