You can add an ssd just fine,

There is no such thing to my knowledge as a 3.5" ssd - you will need an adapter bracket.

Jumping pins is for fooling the thermal sensor on the new 27" iMac. not relevant for your imac.

MCE TECh makes an optibay that will allow you to use a 2.5" ata drive harddrive in place of the ata optical drive. Powerbook G4 optical drive's are the same size and shape as the one in your imac.

Optibay

You Can put a 3.5" SATA SSD in the mac, but you cannot put an IDE hard drive in place of the DVD drive, However, you can put a 2.5" Laptop SATA drive in place of it. The iMac Lacks an IDE Bus, but has two SATA buses, whic can be replaced with one 2.5" SATA drive, and one 3.5" SATA drive

From what I've heard about putting SSD's inside, you have to short two of the pins, and sometimes get unreliable performance.

may I replace the 3.5" standard HD with a 2.5" SSD (I wouldn't like to replace DVD... I use it!)?

Would I need an adapter? Thanks!

Update

One more thing: can i put two 2.5 HD in the 3,5 vane? If I could I would have a small SSD+large HDD... I think I would need an adapter... Does it exist?

Ps: for the moment I bought an ibrid SEagate Momentus (500Gb 140euro) because it was the best for my needs and pockets.

I did it.

I bought an box adapter (2.53 -> 3.5") but it's only to fix it mechanically in the iMac, no need for a plug adapter. Some even does it with tape....

Anyway it's great to have a SSD !

only bug, I taped back (wrongly ?) the thermal sensor on it, and the SSD temperature is now saying always 128° (iStat Menu). But the fan turns at 1300rpm.

Nothing to worry about.