AMD A8-3850 or i3 2100

My current specs are

Pentium D 2.8 ghz
asus motherboard asus pk5pl-am
corsair 450vx psu
msi gtx 550ti gpu
2gb ram ddr2

To eliminate the bottleneck i am planning to buy new cpu,mobo,ram. Which one to choose between Intel i3 2100 or AMD A8-3850?

The i3 would have far better upgrade routes.

A8 if you want to just want to use integrated, otherwise not much point.

core i3 2100 offers larger upgrade path, wider motherboard and chipset choices.
3850 offers strong igp performance, narrower motherboard and chipset choice. upgrade path seems to be locked so far. rumor is, amd will replace socket fm1 (3850's socket) with socket fm2 for future apus in 3850's class. then again, amd is known for backwards compatibility, so you never know.
this becomes easier with your gfx card choice. imo, the core i3 will be able to provide decent cpu performance with your gtx 550ti. if you can hold out till april/may you could even try getting ivy bridge after they launch and get reviewed.

You have a 550Ti so the AMD A series is pointless as though the intergrated graphics are quite good there not as good as a 550Ti and the 2100 is a better CPU

But the A8-3650 rated higher than the I3 2100 in overall testing and remember if your going to overclock then the I3 2100/2120 are locked as well and no hyperthread capable its a tough call there both spec'd out well. and for the same price its a hard one to conclude. So depends on what your going to do with it I guess overall.