Whats your motherboard exactly? There's not many processors in the AMD side to be a huge upgrade for yours, surelly any Quad core will be a direct upgrade, something like this
http://www.newegg.com/Product/Product.aspx?Item=N82E16819113007&Tpk=phenom%20ii%20x4%2095w
Mostly any phenoms ii x4 are upgrades just because the L3 cache and will help solve your bottleneck problem, but you need at least a 125w TDP motherboard to get a real upgrade for your Athlon x3, which can be unlocked to x4 but phenmos are about 10% faster than athlons just because the L3 cache.
A real upgrade should something like an 965 or 970 BE overclocked to about 4.0ghz to take out a bottleneck.
But you can always get new mobo + cpu combo from intel or even from AMD if you want.
EDIT: This is actually the best processor for a 95W TDP motherboard you can get, its Black edition, overclockeable and also unlockable for a 6 core. Only if you mobo supports that. This has Thuban cores which are the strongest CPUs that AMD launched untill today (forget about bulldozer those are at the par of most Ahlons x4 Lol) and these are actually about 20% stronger core per core than your athlon, it will about 60% more power for the 4th core +L3 chache and a good but outdated architecture.
http://www.amazon.com/AMD-Phenom-Processor-Edition-Socket/dp/B0056D5AMY/ref=sr_1_3?ie=UTF8&qid=1338051975&sr=8-3