Buying a 2,5" external FW400 drive only for Photoshop CS2 as Scratchdisc?

Question

i am a retouching professional
and use a pb 15" and a ACD20".
do you think it makes sense to buy a
2,5" external FW400 drive only for Photoshop as Scratchdisc?
2,5" therefore, as it needs no powersupply
by wire. that elegant i guess,
my images have 1,5GB on the disc, and use often 4GB RAM in Photoshop CS2 but i only have 2GB RAM in my pb 15" 1,67 from late 2005.
its very slow sometimes as Photoshop (PS) scratches onto my external drives which are all nearly full of stuff and heavyly fragmented.
so , i had that idea of buying a drive only for PS, which is always "empty" and PS can use it for scratching large data onto the disc. i would format it everyday in order to prevent fragmenting (?) and be able to work fast,
thats my goal, just to work faster. its really slow sometimes as i suffer from too less RAM. of course i am buying a G5 quad later with plenty of RAM but for now i have to work with my pb .... and need a better working solution
anyone thinks it makes sense?
if yes, how big should the disc be, 20GB , 40Gb, or more?
if the file in PS has 4 GB RAM, and has 30 layers or more, how can i find out how many GB space it needs on the scratch disc?
i would like to buy this one:
http://www.sarotech.com/english/cgi/pd.cgi?cmd=view&rno=8
Cutie EX
[IMG]http://i1.tinypic.com/v80774.jpg[/IMG]
if 40GB would be enough i could save more money, but if you say i need 80, its also ok. which model/brand of disc would you put in? i am looking for a quiet one, 7200rpm and at least 40 or better 80GB. but quiet would be important.

Answer 1

thanks for your help.
i had another idea now: i copied my stuff from my internal pb to my external FW 400 disc. now i have 40GB free space in my internal.
- is this enough for photoshop scratch disc?
- should i also define the other 3 discs (external) as Photoshop scratch disc (possible in the prefercenes of Photoshop, up to 4 discs can be used as scratch discs)
- what about the system scratch disc, do both interfere themselves or bother?
- better also copy the image i work with in Photoshop to the internal powerbook drive which has only 5400 rpm or better leave it where it is now on the external FW400 drive 7200rpm?
thanks