Welcome Mikey.
Folks tend to get a little funny with these types of posts as they are asked very regularly by newbies. There are stacks of threads out there you could search that will answer your queries...will keep some of the lads off your back
Anyhow, based on the information you have given (ie) stock auto, exhaust, hi flow turbo, front mount cooler and with a budget of around $500-900 you have a few options and in reality you probably need to do them all..
As mentioned above do a full service and replace any worn bushes, belts, hoses etc. This alone could blow the budget.
Personally I'd get an electronic boost controller (any jap brand is good) and a nistune or similar if you want a boost in performance.
You will probably need to do a fuel pump too... sooner rather than later.
Alternatively you could have a shift kit put in the auto to improve feel/lengevity.
But like I said you'll probably end up doing all of these at some point. Enjoy.