Need some bottom bracket advice?

Hi a have a fantastic but somewhat cheap road bike.. The frame is good (aluminium) and it has carbon forks.. When I bought it I had a limited budget, it's always been my intention to upgrade the compnants as I went along...and it's now got much better wheels and brakes as a result :)

But, while that's all fine it groupset is all shimano 2300 range and is very low end, the chassete and chain now need replacing, but I'm now considering upgrading the groupset :)

Heres my main question... I understand that replacing all the elements should be fine, but im a little confused about replacing the bottom bracket. I have no idea what my current bottom bracket is, but I'd like to replace it with a Shimano 105 BB5700

Can anyone ffer any help Or tips?