Oh yeah, filling your Pokédex in the newer generations is nigh-on impossible as there's about 4,000 Pokémon to collect xD
I think Gold, Silver and Crystal got the balance right; 251 Pokémon is enough to make it a challenge without it being an impossibility. You can complete your dex quite easily in Red and Blue, so long as you have another version to trade with. I wish they'd kept Mew as a prize for when you got all 151 though like they did in the Japanese Red and Green.