This is going to be a bit technical, but I'll try to keep it somewhat simple. Basically, anyone who told you that the OMAP CPU is faster than the Hummingbird CPU fed you some misinformation.
The SOC basically comes down to the three things you mentioned; CPU, GPU, and memory.
CPU: Both use a Cortex A8 MP Core and the ARMv7 instruction set, clocked at 1ghz, and use the 45nm process. The only confirmed differences are some changes to the Hummingbird CPU allowing for 10-20% (claimed by Samsung) speed boost during multi-threading. Don't confuse that with multi-tasking (which when done at times, can be multi-threaded). Overall, they should be about the same, with the Hummingbird being slightly faster.
GPU: OMAP uses the PowerVR SGX 530, and the Hummingbird uses the 540. While we're heard some strange numbers (like 90 million polys), the actual numbers from NEC are that the 540 puts out double the polys and double the fill rate. Hummingbird wins easily.
Memory: The droid X uses 512MB of unified LPDDR2 memory at 200mhz. This means that both the CPU and GPU have full shared access to the 512MB, and the memory is twice as fast as that used in older OMAP and current Snapdragon SOCs (although real world performance is rarely impacted by this memory speed difference due to other system limitations). The Galaxy S uses 2 blocks of memory, one is 384MB, the other is 128MB (total 512MB). In their spec sheets, Verizon lists the Fascinate as having 384MB of RAM. It is largely speculated that the 2 blocks are for system/CPU (384) and the GPU (128). The 384 is DDR (not sure if LPDDR1 or 2), while the 128 is a faster type of memory, which makes sense if dedicated to the GPU.
So overall, what does this mean? System performance should be similar unless you run into a situation that requires more than 384MB of system RAM. If gaming ever really takes off on Android, the Fascinate will be the undisputed champ (between these two). None of these conditions are really expected to happen within the next 12 months to a degree that might cause either phone to really stand out, so you really can just base your purchase off the superficial decisions (UI, form factor, screen quality, etc.).