[Samba] is there a bandwidth limit per file?

[Michael]

Hello Everybody!

I set up a fileserver:
    - Pentium II, 350 Mhz,
    - 256MB RAM,
    - Intel Fast Ethernet PCI
    - VIA-SATA Controller
    - 2x500GB SATA HDD RAID 1
    - Debian 4, Samba 3
    - Samba set up in user shares mode without special options, but with
the suggested optimizations

Clients:
    - Windows XP Prof SP2

So far it works fine. I can read and write files without problem. But my
bandwith seems to be limited somehow. When i copy one file i get a
nearly constant transfer rate of about 6.8 MB/s (read and write).
Do i copy two files in parallel i get a total transfer rate of about
10MB/s (again for read and write)
with 3 files it is even around 11MB/s. Why i can't reach this transfer
rate with just one file? It is the same from both windows pc.

I benchmarked my network around 11.5MB/s are reached using netio
(Win-Win,Win-Debian).
The HDDs are reaching 60MB/s with hdparm -t /dev/sd?

I search for such an issue, but didn't find something useful. I looked 
into the documentation, but didn't find some hints for that issue.

How can i find the bootleneck in the system?

Best reguards,
    Michael