I would like to ask all of you that ASA 5510 and 5520 can do loadbalancing or not?
This is presently not possible with the ASA.
You can accomplish this with PBR upstream on a router.
Two methods to load balance with two ISPs upstream.
1. Add two globals one for each ISP.
2. Translate 1/2 the inside networks to one ISP and the other 1/2 to the second ISP global
3. upstream on a layer 3 device do PBR based on source IP from the fireall.
4. Send globals ISP1 via one link
5. send global ISP2 via the other link
1. you can use just one global.
2. and load balance between the two links to the ISP on the router.