Prototyping an antenna array receiver
1- an RF Receiver is what you want to sit between the Antenna and ADC. Something like the Lime LM6002 or the AD9361 transceiver IC's might make this a lot easier. There are Eval boards that mate up with FPGA eval boards for an easy (Expensive) solution. Do you need to have these two channels to be coherent? 

2- An ADC might have 2 channels (I and Q) of digitized data clocked at 75 or 100 MHz, depending on the bandwidth of the signal you're trying to receive. Perhaps at 12 to 14 bits of bit width, so 1.8GBit/Sec of data to process. This generally goes into an FPGA to do downconversion and digital filtering, with the lower data rate info from the FPGA moved into a DSP for signal processing.

3-You can't really oversample a 2.4 GHz RF signal with a current technology ADC. 

There are several things that might make this easier. Doing this on the FM broadcast band would make this much simpler, as the bandwidths and frequencies are much lower, and easier to deal with. Using narrow band channels for your Experimentation would also help, as that would reduce the RF BW, and reduce the bitrate out of the FPGA. 

(Edited to add TV Tuner Dongles)
Another dirt cheap possibility is to use a TV Tuner dongle which would work for FM broadcast band. Using that and GNU Radio would take care of a lot of the hardware issues,  but wouldn't support the 2.4 GHz ISM band.  Some more details here http://sdr.osmocom.org/trac/wiki/rtl-sdr