I am considering buying a MSI 790XT-G45 and a Phenom II 555BE for overclocking and unlocking. (The new C3 should have a fairly high chance of unlocking, right?) It is supposed to replace a C2D E6600 @ 3GHz. Have anyone tried unlocking a CPU with the 790XT-G45? I haven't found any threads about the board, but it should be able to do it - afaik (due to the chipset).
Are the AM2+ DDR2 boards overclocking impaired, compared to a AM3 DDR3 board?
It has the SB710 southbridge so yes, you should be able to unlock/overclock the 555BE. Having a DDR2 board doesn't mean you're impaired in any way. In fact, I've had far more luck with DDR2 boards than I have with the AM3 boards. I've unlocked all 6 I've tried on AM2+ boards, but I've only been able to do it once on an AM3 board. I currently have an ASRock AM3 board that refuses to unlock the 3rd and 4th cores on a chip that has no problems unlocking on any of the DDR2-based boards I've had it in. That said, I've only tried two AM3 boards so it's not really a fair comparison (compared to 8 or 9 AM2 boards I've tried it in with 100% success). I also haven't tried a 555BE yet - they've all been 550BE chips. The last PC I sold was on a Gigabyte 790X board so you should be good.