Let me pose this question to you: Are you comfortable putting pc parts together yourself? Is it really necessary to get all the "extra" stuff with the computer such as dvd, monitor, windows, keyboard/ mouse?
If you don't need that stuff and are comfortable building a pc you can remove the unneeded stuff and move that money into the basic parts that would get you a better system for mame. Basically just mobo, cpu, ram, powersupply.
Also don't use a 32bit operating system. Make sure you use 64bit... it does have an effect on mame performance.