1) does it really worth buy an ssd hard drive?
a sata 3 120 gb is very expensive how much the machine performance benefit from that?
A good SSD improves your access times and boot up. It has other benefits, but these are the ones you will notice most. Is there a difference? yes. Worth it? That's a harder question. I would assemble the rest of your parts first; IF you have enough in your budget for one, then go for it.
2) is an ati 6950 (that i would flash as an 6970) a much better videocard than mine?
or is just a direct x matter
you have a very decent card right now, but there should be a difference with the newer card
3) i would buy a sandy bridge processor an i5 2500
is gonna be good enough'?
4) what the best chipset to buy for the motherboard that will host the sandy bridge?
i kinda got lost there
The P67 series is considered the high performance mobo
5)does crossfire\sli worth a try?
Personal preference. If you want to try it so you can say you did it and learn from it - yes. Also, in some cases 2 lesser cards (like the 560 Ti) are better than one high end card (580), but cost about the same - so there is some advantage to that. But with your budget, I think your best bet would be to focus on getting the best single card you can afford.
Hope this helps.