The less expensive one would be fine unless you want to use Windows 7 to install Ubuntu by using the Windows Ubuntu Installer also called Wubi.exe on the Ubuntu install disk to install Ubuntu via Wubi.exe to create a dual boot Windows 7 and Ubuntu computer.
You can learn how to use Wubi.exe to install Ubuntu at
http://johnsonyip.com/wordpress/2010/12 ... ller-wubi/
The Advantage of using Wubi.exe to install Ubuntu is it is easy. The disadvantage of Wubi.exe is the disk read and write speeds for your Ubuntu OS would be slightly slower then installing it by booting from the install CD for Ubuntu. However, if you do not like Ubuntu, you can easily uninstall it by using the default add/remove programs program in Windows 7 control panel.
The PC should work fine. I'm running Ubuntu 10.04 on a much slower computer (1.86GHz, 1GB of RAM, DVD-RW, onboard video, 320GB hard drive) then the system you mention, and it is good enough for most of my daily tasks like web browsing, watching HD videos on YouTube or my computer, listening to songs, basic photo editing in Gimp, and office work.
The minimum requirements for Ubuntu 11.04 Desktop Edition are
Processor (x86) 1 GHz
Memory (RAM) 384 MiB
Hard Drive (free space) 5 GB
Monitor Resolution 1024×768
http://en.wikipedia.org/wiki/Ubuntu_(operating_system)#System_requirements
You can bring a long a copy of the Ubuntu Linux Live CD or a USB Flash drive with a bootable version of Ubuntu 11.04 or an older version of Ubuntu like 10.04, 10.10 to test to see if everything like sound, video, keyboard, mouse, and USB works. Also, test to see if the wired or wireless internet adapter works on Ubuntu at going on the internet before buying it if you can.
You can download a copy of Ubuntu from
http://www.ubuntu.com/download/ubuntu/download
and use
http://cdburnerxp.se/ to burn the ISO file or you can use
http://unetbootin.sourceforge.net/ to make a USB bootable flash drive loaded with Ubuntu on it. You may need to mark your USB drive partition as active before you can boot to your USB drive from a computer which can boot from a USB drive.
Learn how to mark your USB Drive's partition as active with command prompts in Windows or Disk Management here :
http://johnsonyip.com/wordpress/2009/11 ... -diskpart/