Again, what is your reccomandation?
You might want to consider a mSATA SSD for your T420s. mSATA SSDs are small SSDs that plug into the miniPCI slot underneath the palm rest. Once installed, Windows sees it as another hard drive. You can put the OS on the mSATA SSD for speed. The benefit of doing this is it allows you to keep the platter drive in the main bay for storage where speed isn't as important. I have this setup on my X220i - the Intel 80GB mSATA along with the 500GB Hitachi Z5k500. It's quite slick. Do keep in mind that using the mSATA drive prevents you from doing WWAN. The WWAN card and the mSATA SSD would share the same miniPCI slot.
I don't think matching memory sticks matters all that much. My X220 and R60e both have mismatched modules and they work fine.
Both the memory modules and hard drives are considered user replaceable and will not void the warranty.
When I got my X220, I made the recovery discs and a copy of the SWTools folder, which contains all the drivers and applications installed on the machine. I installed my mSATA and did a clean install. It's a little more time consuming, but then I get to put on only what I want. I then make an image with True Image that I can put back at any time, then just make any needed updates. If you're interested in a clean install, you can download a disc here
The thing one should be concerned about when using a SSD is whether the partition is aligned. If it's not aligned, performance will suffer. A Windows Vista or 7 disc will align it properly when creating install partition. I don't know if the recovery discs do. It's been so long since I used them. I don't know if Ghost does either. Maybe someone smarter than me can chime in here. If you do a clean install then put it back, at least with True Image, it will remain aligned. That's all I know. Good luck and welcome to TPF.