First thing to do is verify exactly how much upload speed your actually getting forom your ISP (now what they say your getting). Do this by runing a speed test here:
Secondly, your using a slingbox pro and not the current slingbox pro "HD" box wich using a totally different engine and codec You'll get much better results with the pro "HD" model.
Lastely how do you have your slingbox connected to your router? (ethernet cable, powerline adapter, wireless game bridge). an ether net cable would be the ideal senario. If thats the case your router may be the limiting factor effecting the overall preformance.
Accorrding to speedtest.net,
I have upload speeds 600-700kbps (I tested more than once and I got various results)
As for download speed is approximately 5Mbps
I just bought Slingbox pro so I am not thinking about replacing it with pro HD.
My slingbox is connected through an ethernet cable with the router.
Anything else that I can alter for better video quality?
600-700 with varying results as you put it is your bottom line issue. The varience in fluctuation directly effects the picture quality. 768 is the bare recommended minimum.
Also, you said you just purchased the slingbox pro. I assume you bought it used, as it has been discontinued for some time now. You know that is is no longer a supported product right? I'd return it if that's still even an option for you.