Questions about replacing the logic board.

On the most part you can only swept logic boards within the same series. In your case the MacBookPro5,5 which is only two models.

While some have managed to use other series there is always a risk the connections don't line up or the expected signaling is not compatible. Given the few Hz difference I don't think its worth the risks or the costs.

I think you can gain more performance just by upping the RAM as well as a faster storage. But before you go down that path what do you have now?

OK, unless you are doing deep image editing or DB stuff or trying to run lots of apps at the same time 8GB should be plenty of RAM.

The drive its a bit tricky here. Given the systems age I wouldn't put to much money in as you will not likely get it out. I would look at a SSHD like Seagate's or a Dual drive like Western Digitals Black2 drive. Going with a straight SSD while nice, is expensive when you get the to larger sizes. Going with a dual drive setup (HD & SSD) by given up your optical drive only makes sense if the Western Digital Black 2 drive doesn't meet your needs space wise.

But, you still have one issue which could handicap you here the SATA ports speed for both the HD & Optical drive is only SATA II (3.0 Gb/s). So you do need to make sure what you get will run at that speed.

Last issues here is you do what to make sure you are running with the latest system firmware follow this Apple T/N: EFI and SMC firmware updates for Intel-based Macs as well as running the newest release of Yosemite if you go with either the WD Black2 or SSD drive.