Guys give your technical opinion on the optimum size of C-partetion , when:

1. Using Windows7 Ultimate .

2. Having 640 Gb hard-disk.

Also suggest the best partetion tool available in market .
3.

I haven't seen 128GB of Ram, and it's Sandboxie I guess ;)

use a 512GB SSD for C Disk :)

Check this out.
http://www.sevenforums.com/hardware-devices/200431-hard-disk-maintenance-partitioning-question.html

unless you absolutely must have partitions ?
then don't .

if this is dual-boot , and you are adding this 640gb drive
for a second OS .

then have your first OS , format the entire disc ,
and leave it as a new simple volume .

or if this will be your only drive with an OS .

then on the install disc , choose to install to the whole drive .

and i use Casper 7 , for managing disc and partitions .
and for making boot-able back-ups .

If you are the kind that install stuff in the default C:Program Files (or Programs), take it to around 200Gig. Use the D: for programs, games, documents.
Otherwise, 120 is quite enough. (My installation has a 100 C: since 4 years and I'm just starting to run a bit low, mostly because of stupid ITunes).
Unlike a few posts I saw, I would never EVER recommend using a full disk like a 640G in a computer that doesn't have a dedicated disk for the system without partitionning it.

The simple reason is not that it's necessarily bad but it's just a tiny bit better safety net in case of a system crash (not physical since all partitions are on the same disk) and if you play your cards right, reinstalling time can be cut enormously if you only have to reinstall the system and the few obvious things like antivirus and office suite.

If for some reason, you crash hard (still not physically) and have everything on only one partition, you might need to stockpile a lot more caffeine.
Partitioning done right can be done when installing and using the Partition manager from Windows. There are a few partition manager, even free (used one in the past but don't remember the name) and most of them do what they are supposed to do. So, trying to find THA best one is not really a big issue.

For a 640G, I would probably go this way :
C: 120 (System + Antivirus + Office Suite + a few obvious generic things)
D: 200 (Programs + Games + Etc.)
E: The rest (Documents + Movies/shows/etc + Backups of whatever)

Obviously, it's a preference, others manage their stuff differently.

for windows 7 ultimate and 640 hdd,optimum size is 150 gb...and you dont need any tool for partition....just type DISKMGMT.MSC in run...you will see the diskmanagement tool of windows...go there...right click any unallocated memory and you can create new drive....