Software can help but will only do so much.
Game booster works by disabling processes that you aren't using, you can do this manually. I would focus on hardware. Shop around, compare prices and increase your RAM.
To be perfectly honest, Vista is terrible for games, you need a quad core and you need 6Gb RAM. Windows 7 64bit plus more RAM
I personally do not agree with the other two posters; although something like Game Booster can improve your performance, it will not do it to a level that is very noticeable. Other than increasing your current RAM, I don't see any real hardware changes that you can make within your price-range.
I would make sure that your computer is not overly fragmented: this can be done through the Piriform application "Defraggler" found at http://www.piriform.com/defraggler , or through your Windows computer in a default program called "Defragmenter."
On this price range, besides Game Booster I'd say update your drivers(if you don't know the exact names of the parts to search for drivers, use Recuva(http://www.piriform.com/recuva it's from the same company that made ccleaner it's lightweight and easy to use). After updating your drivers, use CCleaner(http://www.piriform.com/ccleaner) to clean some old files and fix window's registry and then defragment your hdd.
Also, if your computer is really slow for some common tasks, you can open window's task manager and google the name of processes that you suspect may be malware.