here's the recommended
Recommended System Requirements
windows xp / Windows® Vista/7 (latest service packs)
Intel® Core 2 Duo 2.4 GHz
or AMD Athlon™ 64 X2 5600+ 2.8 GHz
2 GB RAM
NVIDIA® GeForce® 260
or ATI Radeon™ HD 4870 or better
so basically you would need to change GPU.
You can get a geforce 550 series for about 100 - 130 $ and it would work good, but if you think you will change computer soon you could get something better.
You shouln't be affected by bottleneck
You can go at http://www.systemrequirementslab.com/cyri/intro.aspx
and check what kind of performance you should expect for diablo 3 with you current settings