If your computer meets the recommended requirements (not the minimum) then yes, you should upgrade.
If not then you should buy it anyways and give it to a friend or sell it on ebay or give it away as a prize here.
15 bucks is a great price.
My opinion of the Linux vs Windows vs Mac debate.
Windows = best for average home user
Mac = use it if you have to for work or if it you are already more familiar with it and do not plan to ever be more than casual computer user - Internet, email, flash games etc.
Linux = best if you want to get a head start. In another 5 years it will replace Windows in market share and be the best for the average home user. It is not there yet though.
I agree with Seahawk.
I use
vLite - Windows Vista configuration tool If you can get it installed, it is very awesome.