This comment is based on just a quick skim of your post, while I was watching King of the Hill:
Firstly, those two cards won't run simultaneously, as they're just not compatible. nVidia based GPUs, regarding SLI, are more restricted, whereas AMD (formally: ATI) GPUs can be mixed and matched a lot easier. I believe you won't be able to even use the older card for a dedicated PhysX card, as that card doesn't properly support PhysX, as far as I know - I'm told the earliest card by nVidia (of course) to support PhysX, is the 8800 or something along those lines.
In what I can only assume to be ALL cases, the amount of RAM or CPU power is redundant in order to run the graphics card, however, you will not see the full use of your GPU, if, for example, your CPU isn't capable of keeping up. As the adage goes: "A team is only as good as its weakest member." - or something to that effect.
As long as you have enough power, the correct connections, the correct operating system, and the correct port (in this case a PCI-E x16 port, for optimal use) then I see no problems.
I have a 550Ti kicking about somewhere, and it's a very good card, IMO.
Lastly, there are different types of graphics cards, as you likely are aware, but saying just "550Ti" isn't too helpful, I'm afraid, as there are various forms... such as PNY, MSI, ASUS, etc. When giving that sort of info, include the extra name, because the cards can vary a hell of a lot, such as my graphics card (MSI) currently needs 2 PCI-E 8-pin power cables, where-as other cards, of its model, need only 1.
Good luck, bud.