The main reason some ps1 games will not play on a ps2, is because its not the same hardware. the ps2 doen't have a ps1 inside it. Its a software/programme, that makes the ps2 think its a ps1, so what happens is the ps2 emulates a ps1. For some games this is fine, but for others there is slight changes in the game, heres a few I know:

Metal Gear Solid = On the helicoptor pad (near the beginning of the game) the light from the flood lights is shaded, instead of transparant (like tracing paper, oohhhh...).

Silent Hill = the flash light's beam is shaded instead of transparent.

FF7 (poss. 8 as well) = FF7 on PS2, will not recognise memory cards (well for me it don't) FF8, sometimes the files will be turned into junk data while saving. I think this is because the saves from FF are compressed to save space (well I think they are)

GT2 = I have noticed when I played this on the ps1, when replying racings, the last part of the track dissappeared, the cars raced on thin air!! On the ps2 the whole track is loaded in during reply.

My personal opinion is that since most people who still have a ps1 and own a ps2 as well, keep the ps1 as well the ps2. The ps1 can use the ps2 tv-link up and its power lead so if you want to play on a different one just changes the connections over. Emulators can't beat the real thing, and they are not meant to, they are made for reducing the need to buy different hardware. All pcs are supposed to be x86 (correct me if I am wrong) but if you got that a really old piece of software and used it on a new pc, it won't work as well, as the original computers specs it was designed for. So keep the orignal hardware and if you stick with sony's console, in about 25 years you'll have mini-tower with the ps1 on the top and ps6 at the bottom, mind you'll need a step ladder to reach the top one!! stay faithfull!!

life's a game and I've got an explorer. Infinite lives and ammo, whos going to stop me!?!