That's not how it works, I am afraid, Matthew.
Uninstalling games does not 'give back' anything. Games only temporarily use the resources they need.
SSE will need more than SS08 needs. A 3Ghz processor for starters. If your machine has a 2.0Ghz processor and it runs SS08 fine, then you're just lucky that the rest of the configuration allows for this.
But it doesn't mean that SS08 'uses up' 0.4Ghz of what the internet says you should have, and returns this to you if you uninstall. It can mean that maybe there's more than one hardware configuration possible with your type/brand laptop. You better trust the dxdiag, and dont go by what a website tells you. Your own computer knows what it has onboard.
You'll need a minimum 3.0Ghz P4 processor for SSE, so you can pretty much forget about that with a 2.0Ghz cpu, I am afraid. This games will up the ante quite a bit, compared to SS08's minimum requirements, as you can see. And you need to have that whole set covered, pretty much. There's always a lucky few that can run just under specs, but I wouldn't count on it if you're too far below them.
But just so you know, installing and/or uninstalling games does not change how fast your CPU is, or how much RAM you physically have installed in your machine. It only used resources when it runs, but never changes any of your hardware specifications.
Regards,
Fred