From a basic resiliency standpoint and lowest used ports & cables I'd consider the following, if the devices support spanning tree:
Switch A Port 1 -> Router Port 1
Switch B Port 1 -> Router Port 2
Switch C Port 1 -> Switch A Port 2
Switch C Port 2 -> Switch B Port 2
Set spanning tree on for those ports only. This way you'd be fine when(not if) any one of the devices/ports/links fails.
I can't speak to American pricing, but if there is a HP ProCurve Switch in your price range then you might want to take a look. I've found their products to be keenly priced (in the UK anyway), great performers, and very well supported products with a very good guarantee.
I'm not personally a fan of DLink in general but can't comment on the specific model you are looking at. Regardless of what model you buy in the end you have the right idea with going from a group of 'home' products to a business class switch, but also consider the environment - clean power, well ventilated, decent cabling will all help...
Your proposed setup looks fine to me, I think you'll certainly see an improvement in reliability and throughput by reducing the quantity of switches and improving their quality. Simple is nearly always better.
You do have to consider that any product, no matter how well built, how well recommended, how well cared for can fail occasionally. If an outage is unacceptable, then whatever product you buy then it might be worth trying to find the money for two in case of a problem. Whether or not the risk vs. investment makes sense for you is something only you can decide.
I can say that from running a major business network, just about every make of 'business class' switch I've used has been very reliable - as long as they're not dead on arrival they should last for a very long time. The ProCurve switches have had the best record in this area for me, which is why I like to suggest them to others.
Best Answer
Best thing to start with is setting up a Trunk Port on one of the four last Ethernet ports, as you will need to Tag all our VLANs out to the next switch. could be best to buy the same vendor (DLINK) as i've had to do a few firmware upgrades on DLINK's myself to get them to properly tag packets for and trunk
It also sounds like you want to setup an Aggregated link, this is possible with this switch but only in "Static Link Aggregation" mode, if you've got a Cisco 3700 handy you should be able to set it up using LACP
best to have a quick read of this; http://en.wikipedia.org/wiki/Link_Aggregation_Control_Protocol
btw.. the DGS-1248T has 4 SFP ports, not sure it's mini-GBIC ports on that device