Don't be afraid to ring them up and ask all the nitty gritty details if you're unsure.
With regards that specific machine.
It is already PCI-Express.
It WILL work first time out of the box and could saves hours of frustration setting up your own PC.
It will take a minimum of a 3.6GHz in the future.
Dell build is usually good to very good and accessibility usually excellant.
The 17" TFT is the Dell E173FP. Can be seen on Dell and Aria's website for sale separately. We have some here and for windows use they proved far better than we expected. Not games tested by me however.
The 3yrs NBD warranty is on offer. True any components you buy should have at least 1yr but that means striping the machine if it fails. Dell will come to your house!
In other words it's reasonably future-proof (PCI-Express etc).
Good value.
Easy.
Building is more fun and for someone upgrading is probably a better option. However in basic spec that Dell is very hard to beat!