Generally the biggest bottleneck is from the CPU, not necessarily the PCI-E 1.0 slot
We usually say the slot will bottleneck because the CPUs with those boards will be the main thing holding it back since they are older, weaker chips
Running in x16 mode, there isn't a giant difference
There isn't anything bad from bottlenecking, you're just getting less value for your money that way unless you're planning a CPU+mobo+RAM upgrade in the near future