I have changed the configuration. see if its ok

interface Dialer1
mtu 1480
bandwidth 6000
ip address negotiated
ip nat outside
ip virtual-reassembly
zone-member security outside
encapsulation ppp
ip tcp adjust-mss 1400
dialer pool 2
dialer-group 2
ppp authentication chap pap callin
ppp chap hostname *****
ppp chap password 0 ptcl
ppp pap sent-username ****** password 0 *****
no cdp enable
exit

interface FastEthernet8
ip address 192.168.8.2 255.255.255.0
ip nat outside
no shut
exit

ip sla 8844
icmp-echo 8.8.4.4 source-interface dialer1
frequency 10
exit
ip sla schedule 8844 start-time now life forever
track 1 ip sla 8844
track 1 ip sla 8844 reachability
delay 10 up 10 down 10
exit

route-map for_isp1 permit 10
match interface dialer1
exit

route-map for_isp2 permit 10
match interface FastEthernet8
exit

ip route 0.0.0.0 0.0.0.0 192.168.8.1 2
ip route 0.0.0.0 0.0.0.0 dialer1 track 1

ip nat inside source route-map for_isp1 interface dialer1 overload
ip nat inside source route-map for_isp2 interface FastEthernet8 overload

also should i add these lines ???????
ip access-list extended for_isp1
permit icmp any hotst 8.8.4.4
permit icmp any 8.8.4.4