Hello Nasr,
be aware that without knowing more details about your needs what can be said here has limited value some notes are possible:
the use of GRE + VPN is becoming common as reported by other collegues and may allow for consistent savings.
This can be a good solution if BW requirements for remote site to central site connectivity are not high (1-2 Mbps is fine if you need 10 mbps or more and you want to grow more you should think of VPLS or MPLS L3 VPN solutions, availability and prices depend from zone to zone).
The idea to put each regional site in a dedicated non-zero area allows for fine routing control.
The drawback may be scalability: each OSPF area even if totally stub has its own link state database to build and to keep updated.
So here this can be acceptable or not depending on the number of regional sites involved: if they are 10 to 30 it can be a good idea otherwise this simply doesn't scale.
if they are more it can be reasonable to put groups of them in the same area accepting to receive routes of other remote sites in the same area.
There are specific designs and technologies that are of great help if the number of spokes is high:
DMVPN and more recent GET VPN should be considered.
About DR and Central Site:
I wouldn't put EIGRP into the picture it makes things unnecessary complex.
Depending on the type of links there are solutions to use them also with OSPF (it is enough to find a way to logically divide the faster link in two logical links if speed ratio is 2).
also you need to think at the backup needs:
all remote sites should have a "backup" VPN terminated at DR site to be used in case of failure of primary tunnel.
if using a single OSPF domain DR site should be in area 0 as central site to avoid headaches with virtual tunnels.
This is just to give some suggestions that may be helpful or not.
Hope to help
Giuseppe
