Small Business Network Switches/General network configuration
Solution 1:
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.
Solution 2:
I've used the exact DLink switch you've referenced in your question. My experience is that they're reliable, but the web interface is buggy and troublesome. In some cases, it refuses to allow you to log in until you power cycle the switch. If you can live with that, then they are a reasonable choice.
We've moved to entirely HP switches now.
Other than that, your diagram looks good.
I would consider using the WRT54G in bridged mode (perhaps with DDWRT) and connect it to the switch using one of its 4 "internal" ports. The way you have it wired in your diagram, you may end up with an additional layer of NAT which you probably don't need.
Solution 3:
Ridiculous crap you have so far ;) Here is how I solved that (being in a similar situation):
NetGear has a nice small passive 16 port switch. Stackable. 1gbit - forget the 100mbit crap those days, every motherboard of the last years supports 1gbit anyway.
The good thing on that one is that it is SMALL, SILENT (no moving parts) and METAL.
Never looked back ;) The other lower end products looked - hm - "cheap". This one at least had the money for a sound metal outside.
Check http://www.netgear.com/Products/Switches/DesktopSwitches.aspx
I have the GS 116, and will soon get one or two smaller ones for in room cabling (stupid me having 4 computers on my desk now - easier to just run one cable into the room (the others are currently lying on the ground) and use an internal switch then).
You normally do NOT need managed on that level ;) Seriously ;)