Postgres benchmarking with pgbench

Hello List,

i would like to pimp my postgres setup. To make sure i dont have a slow
hardware, i tested it on three diffrent enviorments:
1.) Native Debian Linux (Dom0) and 4GB of RAM
2.) Debian Linux  in Xen (DomU)  4GB of RAM
3.) Blade with SSD Disk 8GB of RAM

Here are my results: http://i39.tinypic.com/24azpxg.jpg

Here is my postgres config: http://pastebin.com/m5e40dbf0

Here is my sysctl:
----------------------------------
kernel.shmmax = 3853361408
kernel.shmall = 99999999

Here is my pgbench benchmark script: http://pastebin.com/m676d0c1b



Here are my hardware details:
========================

On Hardware 1 + 2 (native linux and debian) i have the following
Hardware underneath:

--------------------------------------------------------------------------------------------------------------------------------------------------------
- ARC-1220 8-Port PCI-Express on Raid6 with normal SATA drives 7200RPM
- 1x Quadcore Intel(R) Xeon(R) CPU   E5320  @ 1.86GHz

The Blade:
----------------------------
- 2x 16GB SSD set up in striping mode (Raid 0)
- 2x Quardcore Intel(R) Xeon(R) CPU  E5420  @ 2.50GHz


Any idea why my performance colapses at 2GB Database size?

Thanks,
Mario