1. Can we reinstall mac osx again after some months of usage , like i use to do on my windows pc for a fresh new pc look
2. And if i upgrade to new os example Mavericks , then can i later downgrade to mountain if system starts running slow ?
1. Yes of course. What a strange question.
2. Unlikely. Why would the system run slow and why do you think downgrading your OS is a solution to performance issues? If you have any doubts about 10.9 just wait 6 months or whatever until 10.9.1, 10.9.2 etc are available to fix things.
4GB is the absolute minimum. If you can't get the 8GB option then I would re-consider your purchase. I wouldn't be happy buying a new machine in 2013 with just 4GB on-board...
1. Can we reinstall mac osx again after some months of usage , like i use to do on my windows pc for a fresh new pc look
2. And if i upgrade to new os example Mavericks , then can i later downgrade to mountain if system starts running slow ?
1. You can, but it's not necessary. OS X isn't like Windows. You don't need to wipe your data/format the HD/reinstall everything every year because the OS just starts sucking; OS X cleans itself up.
2. Yeah, but why in the world would you do that?
To answer #2, software incompatibility. I use apps that don't work as well in 10.7/10.8 and since it's a fairly niche market with small developers it's taken a while to get that compatibility.
So if your software says it'll work in 10.9 mostly, you may have to try it and see if it works for you. This has happened to me and keeps me on older versions of OSX.
2. And if i upgrade to new os example Mavericks , then can i later downgrade to mountain if system starts running slow ?

To answer #2, software incompatibility. I use apps that don't work as well in 10.7/10.8 and since it's a fairly niche market with small developers it's taken a while to get that compatibility.
So if your software says it'll work in 10.9 mostly, you may have to try it and see if it works for you. This has happened to me and keeps me on older versions of OSX.
1. Can we reinstall mac osx again after some months of usage , like i use to do on my windows pc for a fresh new pc look
2. And if i upgrade to new os example Mavericks , then can i later downgrade to mountain if system starts running slow ?
The only option then is retina mbp for 400$ more
You can get any of Apple's laptops with 8GB of ram...
The only option then is retina mbp for 400$ more
You can get any of Apple's laptops with 8GB of ram...
It's a built to order option on the MacBook Air so might not be available in all countries.
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You could also go for the non-retina MacBook Pro and upgrade the RAM to 8GB yourself.