hi jay
your spot on ... alienware laptops have whats known as "MXM" modules which are small expansion cards ( with a graphics chip on it ) which go into the specific slots built into these laptops .... however not all MXM modules are the same so on may not fit another ,infact i have one of these mxm modules ( in the shape of a geforce 8600M ) which was scavenged from a old laptop( not working ) i was given .....
however your HP Pavilion dv7-4270us is a pretty good gaming laptop ( as laptops go ) and from reviews ive read it will run games such as crysis ( which is good for desktops let alone laptops ) ... so what games are you having problems with ( or do you just want better graphics )
one option would be a external graphics card such as the "vi-dock"( http://www.villageinstruments.com/tiki-index.php?page=ViDock ) which allows you to use a desktop graphics card via your laptops "express cardslot " ... then the graphics card is installed in a external enclosure and is also powered externally ... however there are 3 versions with the vidock 3 allowing for cards requiring up to 75w of power ( eg HD7750,geforce gt630 ) ... the vidock 4 supports cards up to 150w of power ( eg radeon HD6850,geforce gtx550ti ) and the vidock 4 plus supports most high end card up to 225w ( eg radeon HD7850 and geforce GTX660 ) ..
the vidock does not come cheap with the vidock 3 costing $199 and the vidock 4 plus costing $279 ( http://www.villageinstruments.com/tiki-index.php?page=Store ),however ive seen the vidock 3 in action and i was pretty impressed .... and it was running demanding games with a mediocre laptop ...... pretty expensive but the best option for laptop owners
in summary you cant upgrade your laptop as it is .... however the ATI Mobility Radeon HD 6370 which your laptop currently uses is a decent mobile graphics processor and with some ingame tweaking you should be able to run most games at playable frame rates ... however for superior performance the vidock range would be your best bet ...... ( note : the vidock still requires a graphics card so as you can imagine the complete package including a graphics card could cost up to $400 )
i hope this helps .. any questions let me know
good luck mate !