Only the southern part of Serbia uses Cyrillic, and even there they are Roman transliterations. If you're stopping at Belgrade you will see very little. You will unfortunately also be missing out on a very empty bit of road that runs between Belgrade and Nis.
Budapest - Belgrade; knowing the history of G3000, and having done that trip, that route is just going to be absolutely insane. Despite what you might think on some maps, from the Serbian border down to Belgrade Sad is mostly single carriageway (as a half built motorway) - I uploaded some pics in the international section if you want to have a look. It's starts off OK, but gets busy as you get further south - and here they eventually re-mark the road from a hard shoulder & two narrow lanes into two wide lanes; this is informally used as 4 lanes, with a 5 suicide lane. It's not uncommon to see a car overtaking another car, which is overtaking a truck, and exactly the same happening coming in the opposite direction. Somehow everyone seems to pull back in and all fit through.
It was however the most fun driving I've ever had the experience of doing.
Watch out as you get near to Belgrade. The signing is a bit dodgy, and it's easy to miss the exit - you can do a bit of google recon here:
http://maps.google.com/?t=k&om=1&ll=44. ... 6,0.010064. You'll be coming from the north; looking through the G3000 forums there's a stop in Central Belgrade, so keep a sharp eye out for a sign to Nis and it should get you in the right direction - I don't remember Belgrade itself being signed here, only Nis.
Would be nice to know if they've finished upgrading the M5 in Hungary from Budapest to the border yet, when I went it stopped about halfway - hell, would be nice to know if they've upgraded the border, was having shiny new buildings built back then, the border I went through was scary in a Soviet kind of way.