Chad, have you engaged your Cisco account rep to have a sit-down design session with a CSE to evaluate the price concerns, oversubscription rates, etc? This is something they can usually work with you on, especially if you're going to be buying a chunk of gear at one time.
As we said in the other thread, the model you use to accomplish your goals are not as relevant as if they work. The 6500 chassis is just a frame, the Sup you plug in is the deciding factor of it's capabilities.
Since you already have 4500s - could you trade in the Sups you have in those for lesser Sup cards and move those to the access layer?
As the CCDA suggests, look at the business needs and then factor in the business and technological constraints. If you determine that there is a technlogical constraint with this design, you should at least throw it out on the table. There are pros and cons to either solution. I have used a 6513 with a supe 32 as an access layer switch before. There are power differences that I had to consider and BTU issues I had to consider. Look at your constraints. That should help you with your decision.
I have not engaged any Cisco resources other than a few CCIEs. I have not asked about trading in the SUPs, but my thoughts are if I go with 6500s using SUP32s then they 4500s with the SUP7E at the distribution layer would be good to go. What do you think?
Also, do you know of any good post related to Switching backplane, to help me better grasp it?
Chad, I recommend engaging a Cisco resource. SEs and CSEs are paid to help out customers in how they spend money with Cisco(and of course, encourage it). If you know you'll be buying gear, engage your account team(or have Cisco assign you one) and work with them. I know the feeling of "I can do this, I just need more info, but I can do it". Well, consider Cisco your "more info". They can confirm if you're following best practices(which help with TAC support down the road), they can provide insight into product life cycles, road maps, where the product lines are going, etc. They can ensure your network is growable for the long term. I don't work for Cisco or a Partner, so this advice earns me nothing, but I cannot stress how helpful Cisco has been in some projects I've been involved with in the past.
As far as switching backplane, you should refer to the product page for the product in question. The backplane is basically the internal bandwidth a device has to move data between the CPU, line cards, feature cards, etc. Backplanes exist on the chassis, Supervisor cards, line cards, they all have limitations to take into account. To further complicate matters, you need to identify if things are fabric-enabled on some of the older models(most newer models are IIRC). You need to take backplane capacity into account, as well as per-linecard-capacity in relation to your port oversubscription model.
This is why design usually involves so much research! You need to know where you're at right now, where you need to get to in the near term, and where you plan to be in 3-5 years(depending on your corporate lifecycle policy).
I would have to also suggest the 2960S series as they are stackable like the 3750 series but don't have the same L3 capabilities. If you don't need L3 then this might be even better value than a 6500. They are great switches with good density (I have two). You can add the stacking in later (it's modular) if you want or get it at the begining.
Worth costing up.