You mentioned the PA1X and not the PA1X pro ( the pro has 76 keys)
The Pa1x has several advantages over the PA800
The sliders are very important in live play which means in real time you can adjust volumes within accompaniments and instrument types on the fly. If you play any form of drawbar organ, again you can adjust the drawbars in real time on the fly. These functions are impossible IN LIVE PLAY on the PA800.
The speakers on the PA1X are simply superb and they alone validate the extra cost compared to the inbuilt speakers on the PA800 which are ok but they simply cannot compare.
Lastly a word about expandability.
I have on my PA1X the xprodata piano expansion card which brings the PA1X piano sounds above (in my view) the new pianos of the PA800 ( which i also have loaded on my PA1X incidentally for free from Korg).In effect i now have over 200 very high quality accoustic and electric pianos from concert grands,bosendofers,and upright pianos to suitcase rhodes, DX7 and the like. All at the touch of one button And i would like to point out that none of the expansion boards affect the onboard sounds of the default factory settings when the expansion board is not selected.
Further there a second expansion slot on my PA1X that i am seriously looking to use. Again its an expansion card from from Xprodata that in effect changes your PA1X at the touch of a button into a whole new keyboard at a cost of less than a 5th of the price to buy a new PA800. This expansion board completely reworks ALL the current styles on the PA1X and provides hundreds of new styles and hundreds of new sounds using brand new samples and drum kits. The new sounds are automatically integrated into the existing styles of the PA1X and are equalised and programmed to get the best sound using the newer samples. It also has an intelligent function that will automatically assign the most appropriate sounds to any midi file that you might use onboard the instrument . The benefit of this is that you are nolonger forced to work with the standard GM sounds that are usually assigned when you play a midi file, with no additional tweaking your midi files sound at their optimum best from the get go.
This is not available for the PA800 currently but i know that there are likely to be some developements in the future from 3rd party developers.
Let me say outright that i have no connection either with korg or with Xprodata. I am simply an enthusiast just like most of the people here on the forum.
If you buy either the PA800 or the PA1x/pro you might want to have a look at this training DVD of which i am one of the authors.
http://www.korgpa.com/pa_root/en/support/arranger_secrets.html