You generally get what you pay for. Spending more money doesn't necessarily mean better performance but if you buy a computer in a department store w/o knowing what's inside, expect to be disappointed.
As to the OS question, in the history of computing, upgrading to a new Microsoft OS has never made a system faster w/ two notable exceptions:
Vista -> Win7 (gain is small but measurable)
W4WGs -> NT4 (assuming adequate memory, you gained about 10%)
"Worse Upgrade Ever" title goes to W4WGs -> Win95 which resulted in an average 37% performance drop.