Been using Mavericks since public launch, many improvements inc better cloud connectivity and keychain, tabbed finder, imprived Safari, improved memory management. Not sure why you think machine is slower bs 2 years ago. For most normal use between 4 and 8gb ram seems optimal. I put a 750gb HDD Western Digital Scorpio Black running at 7200 rpm vs older disks at 5400, very smooth. I needed 500gb min and at £65 vs 250 for an SSD I chose the HDD which is good enough for me (2009 Mac Mini). If you get a new disk put the old one in an enclosure (useful for switching anyway) and use for extra stroage / time machine. As I post on every thread have a browse of MacRumors forums, they are excellent.
Good luck.