are you planning on using the GigaStack modules ? If so, you must use one of the Cisco proprietary cables (CAB-GS-50CM or CAB-GS-1M). If you use any other cable, you will not have connectivity.
Also, keep in mind that the 2950 only operates in half-duplex mode when you cascade it (at 1-Gbps forwarding rate). 2-Gbps full-duplex connectivity can only be achieved in a dedicated, switch-to-switch configuration.
Acutally the 2950C switch (2950C-24 to be precise) is fixed configuration 24-port 10/100 and 2 port 100BaseFX swtich. Therefore, you cannot stack these switches using the Cisco x3500 stack modules or the CAB-GS-x stacking cables. You can either do a cascade connection via the fiber ports using a 1-meter duplex multimode fiber patch cable or you can do a cascade connection via the user ports using a crossover UTP patch cable.
If you ask me, go for the cascade over the 10/100 ports (if you have enough ports left after you've connected your users), you can also create a fastetherchannel for a greater bandwidth between the switches.