This might be a bit long winded, but I'll try to address most of your issues in one go. Wish me luck!
You can have your upstream service providers advertise a default route (0.0.0.0) through BGP to your routers. When you configure OSPF on your BGP routers, you can use the command "default-information originate." As long as you're getting the default route from your ISP (which only happens when you have connectivity to that ISP), you'll advertise the default route into OSPF. As soon as you lose that default route from your ISP, you'll lose the OSPF default route.
You'll need to establish BGP peering between all of your internet routers (the ones connected to your ISPs). This is an internal BGP or iBGP relationship and needs to be a full mesh. Per your diagram, you only have two internet routers, so a full mesh is simple. There are no tricky steps to peer on iBGP versus eBGP; just a simple neighbor statement. When BGP sees that your neighbor has the same AS number, it forms an iBGP relationship instead of an eBGP one.
Preferring one route over another is a bit tricky, particularly where BGP is concerned. Load balancing is done using route maps and traditionally with as-path prepending. There are a few ways to do this. I'd like to add a caveat that if you aren't filtering outgoing route advertisements, you'll end up advertising ISP A's routes to ISP B and C and so forth which will turn you into a transit AS and you'll end up piping some A <-> B, A <-> C, and B <-> C traffic through your network which is probably not what you want.
Here's one go at your load balancing:
! Set an IP access list that matches your BGP-advertised network
ip access-list standard 1 permit a.b.c.d mask a.b.c.d
! Set an IP as-path access list to only allow advertising of YOUR network
ip as-path access-list 1 permit ^$
! Make a route-map for ISP C, match the as-path and access-list above, then make it look
! less appealing than going through ISP A/B
route-map ISP_C 10
match ip address 1
match as-path 1
set as-path prepend <Your AS> <Your AS> <Your AS>
! The more times you prepend your AS to a route, the less desirable it looks, so traffic
! will be more likely to come in via ISP A/B than C. The last step is to add it to your
! ISP C neighbor statement in BGP
router bgp <Your AS>
neighbor <ISP C> route-map ISP_C out
If you have more than one subnet, you can even things out a little more and use router B for site B and router A for site A by selectively prepending your AS path to individual subnets. Here's an example:
Router A:
ip access-list standard 1
permit <site B subnets>
ip as-path 1 permit ^$
route-map ISP_AB 10
match ip address 1
match as-path 1
set as-path prepend <Your AS> <Your AS>
route-map ISP AB 20
match as-path 1
router bgp <Your AS>
neighbor <ISP A> route-map ISP_AB out
neighbor <ISP B> route-map ISP_AB out
Router B:
ip access-list standard 1
permit <site A subnets>
ip as-path 1 permit ^$
route-map ISP_C 10
match ip address 1
match as-path 1
set as-path prepend <Your AS> <Your AS>
route-map ISP C 20
match as-path 1
router bgp <Your AS>
neighbor <ISP C> route-map ISP_C out
What you've effectively done is make site B's subnets look less appealing coming from Router A than they do from ISP C and make site A's subnets look less appealing coming from Router B than they do from ISP A/B. You might have to play around with your AS path prepending some to get the right amount of prepending in.
I hope this helps! BGP is a bit of a monster, but once you understand the parts, it's fun to play with. I highly recommend the CBT Nuggets series on BGP if you feel a bit shaky on the subject and I always use GNS3 as a testbed to verify big network changes before I implement them.
Are you running BGP with ISPs? Or just defaults towards Internet?
You should propably configure HSRP on the internal side between routers to have single default gateway for your network - or multiple default gateways if the inside will be segmented in VLANs.
If you're running BGP and you do have PI space, announce it on both links, then try to tune using the BGP traffic engineering features.
If you don't have BGP, you can try to use Performance Routing coupled with NAT, because you'll need to match outgoing traffic with properly selected source IP addresses (your ISPs should do uRPF and check what source addresses you're using on links they provide you).
As Mike stated, topology drawing would benefit the discussion.
Best Answer
Taking into consideration that you haven't posted your switches makes and configs, your WAN link could be your bottleneck.
While saving on some costs it is usually a half-duplex connection.
Recommendation:- For the desired bandwidth between your sites, it makes more sense to rent fibre (or sdsl) from a Telco/ISP provider as a point-to-point between your two sites (or install your own fibre).