RTS threshold, fragmentation, and other advanced WiFi settings
2346 is the maximum 802.11 frame size. Setting the RTS and fragmentation thresholds to the maximum means that no packets will meet the threshold.
The fragmentation threshold limits the maximum frame size. This reduces the time required to transmit the frame, and therefore reduces the probability that it will be corrupted (at the cost of more data overhead). The RTS threshold specifies the frame size at which the transmitter must use the RTS/CTS protocol, which is largely to solve the hidden node problem. This obviously also adds overhead.
Not necessarily -- if you don't have a hidden node problem then changing the RTS threshold won't improve performance. In order for RTS/CTS to kick in the RTS threshold must be the same as or smaller than the fragmentation threshold though.
I would start with setting them such that a standard Ethernet frame is fragmented into two 802.11 frames (1500/2 = 750 bytes payload + 34 bytes overhead = 784 bytes) and any frames bigger than a third of a standard Ethernet frame uses RTS (534 bytes).
As far as I know though, both these settings only affect the transmitter, i.e. configuring them on the AP only makes the AP use them for its transmissions and doesn't make the clients use them for their transmissions.
That mixed b/g scenario is particularly suboptimal. You may want to review some of the previous discussions on the topic, such as:
Slowest wireless client dictates the connection quality of all others?
Also, another performance killer occurs when point A can receive point B's signal, but B cannot receive A's signal. Someone else on ServerFault pointed this out as the "hidden transmitter effect". More about that phenomena at the link below. They point out that:
"...While horizontal polarization is desired, the lack of inexpensive commercial horizontally polarized omni-directional antennas may require the use of vertically polarized antennas. A good omni- directional vertically polarized antenna will cost about the same as a parabolic antenna. Use of an omni-directional antenna helps minimize the "hidden transmitter" effect. "