I'm currently have 3 mb (PC-5300).
I think you mean GB.
The new memory would only run the lower stated speed if mixing modules (If the MOBO accepts it). You would notice no differnce unless you like running benchmarks.
How many of which RAM modules have you got installed already? 4 slots in XPS 720?
A few points.
Firstly, you woud want the 64bit Windows 7 as 32bit can only use 3-3.5GB.
Secondly, you only have four slots, so upgrades would have to replace current modules depending on the current set up.
Thirdly, You may not notice much improvement unless you are performing memory intensive tasks.
Hope that helps.