My actual setup:
- motherboard: Asus F1A75-M LE (NOTE: it has a 24-pin EATXPWR + 4-pin ATX12V connector)
- CPU: AMD A6-3500 (stock voltage and frequency)
- CPU heatsink: AMD stock
- case: NZXT Nemesis
- RAM: 1x Corsair CMZ4GX3M1A1600C9
- 1x 2,5'' HDD: SEAGATE FreeAgent GoFlex Portable 500Gb (connected via SATA)
- 1x 3,5'' HDD: Samsung EcoGreen F2 Hd103si 1TB
- PS/2 mouse and keyboard
- 1x USB SD card reader
- 1x USB gamepad
NOTE: no discrete video card plugged

Is it ok if i get an 80/90W PSU?
Is it ok if i get a PSU with a 20-pin connector instead o 24?

Here's Anandtech's review of a Puget Sound PC with the A6-3500 and an 80W brick. You have two 3.5" drives which will consume a few watts more than the SSD and 2.5" drive in this setup. So, 90W should cut it..but you might consider 100W for some breathing room.

Thanks for pointing me there, the case used by the Purget System is an Antec ISK-110.
A 20-pin plug should be fine since i won't exceed the 10A ATX spec on each rail.
My only concern is the 3,5'' HDD may exceed the PSU rating for the 12V rail, so i'm trying to estimate that:
2A is required for the drive spinup + 5A for the rest of the system, so i guess the picoPSU-90 is not fine (rated 5A max).

Since it won't be installed in a space-constrained case, do you think using a bigger PSU board like this will help dissipate the heat better?

With the PicoPSU the 12V is a passthrough of the 12V input if I remember correctly. Therefore it is down to the size/quality of the AC/DC power brick which will influence this.

In the "-WI" models the 12V input is not passed through. For the other models the manuals still report different max load ratings for the 12V rail:
1. PICOPSU-90 -> 5A
2. PICOPSU-120 -> 7A
3. PICOPSU-120-WI -> 6A
4. PICOPSU-150-XT -> 8A
I guess i should be fine with 2 and 4.

EDIT: the Winmate DD-24AX looks even better (rated 10A max)!