the modem may be a slower type. A modern internal one can be easily bought with change from a tenner...
click here or click here
Certainly your procesor is adequate for what you suggest. Gaming, video editing, serious sound manipulation, and big picture editing are the demanding tasks required of pc's in a domestic environment. Ordinary "office" tasks are not a problem.
I've just had good service from click here and I suspect they will be able to help with memory. Also a phone call to crucial.com would not go ammiss. Though your mobo is old they are very helpful and may be able to identify exactly what you need, even if they can't supply it new.