Hi Guys,
Welcome to apature fever!!
Rule of thumb for a scope is 50 x inch of apature is max magnification. Your SW500 therefore is capable of 300 x magnification ON A REALLY GOOD NIGHT. So for your 750mm F length the smalles eyepiece would be 2.5mm (or 5 with the 2 x barlow).
After that you have to start looking a a bigger mirror
More of your money spent *cough*
VAZ