Hi,
I'd start poking around and seeing what services are running which you don't need - check 'Services' in Mandrake Control Centre.
You could also have a peek at what's running at the moment and what sort of load the system is under when 'idle' - open a text console and type
.. for a list of processes running + cpu load, etc.. (the greedy stuff will be near the top of the list - hence 'top')
750MHz won't be lightning quick, but it should be more than acceptable - even using KDE (I've recently installed Mdk 10.1 / KDE on an ancient Celeron 300 and performance was reasonable - quite surprised me)