Motherboard: Biostar A780L
Processor: AMD Sempron 140 unlocked to Athlon II x2 440
Graphix Card: Amd Radeon HD 1GB 6750
RAM: 2GB DDR2
A generic 600W Power supply which I want to replace with a branded FSP Hexa 520W
Originally, I am planning to buy a Phenom II x4 955. But I found out that it has been phased out for almost a year now. All I have found so far are Athlon x2 Dual cores. I can't believe it.
Right now, I have a Sempron unlocked to Athlon II x2 440. So, I though that unlocking Phenom II x2 555 to Phenom II x4 B65 is possible for my mobo. But I want to hear from you guys especially about safety measures. I am a newbie here but I am aware of the possible consequences that I can face after unlocking. The unlocking of Athlon II x2 440 came from just a video turorial. I hope you understand.
If it's not really recommended to unlock it, will the Phenom II x2 555 alone will suffice for playing AC3? I am also staggering for the low fps issue.
Unlocking is never a sure thing; it may or may not work. That you've been able to unlock one chip on your board is promising, BUT make sure it supports the additional current used by an unlocked CPU. For example, if your board supports only up to 95W CPUs, you do NOT want to unlock an 80W CPU into one that draws 125W; you could risk blowing the VRMs on your board.
I think a Phenom II X2 should be able to play AC3, but undoubtedly not as well as a X4, or as well as an Intel i3 or i5. Your graphics card is an additional constraint; a HD6750 is not a very powerful card.
If you can provide a budget, we can make suggestions on how best to use it. If you have $120 or so (above the cost of the new PSU), a HD7770 would help. With $150, a GTX650Ti is another step up. With $180, a HD7850 is probably the most that's worth spending on graphics before replacing your CPU/Mobo/RAM; you're more in the $400-$500 range at that point, which may or may not be feasible.
Edit: misc. spelling and grammar.