Versailles
 

Should I book tickets to Versailles in advance?

yes, you can

Only if you want to avoid horrendously long lines.

does it take to working days to process the tickets? cause i'm leaving sunday....?

Another option is if you are visiting 4 or more museums in 2 days, you should be using this anyway http://en.parismuseumpass.com/ which gives you straight access to entrances. Versailles plus Trianon, if purchased by themselves are 15€ and 10€. The 2 day pass is 42€ and let you skip ticket lines not only at Versailles, but also at Louvre, etc. You can buy these passes at any participating museums (but at a less visited one), FNAC, TI at airport, etc. It seems to be sold everywhere.