I have to say this... you should consider building your own PC. It's not as difficult as one might think and you'd either save a bunch of money or get a far better system then you would if you went with anything Alienware could give you. Either cheaper price for same performance or better performance for the same price, depending on which way you prefer to roll with it. Okay, enough of me telling you what you should do with your money (lol).
For ME there isn't a huge amount out there. Though I'd feel foolish NOT to mention MEUITM. Its a fairly huge texture improvement mod for the first game. It even comes with an installer that installs it all for you so you don't have to mess with texmod or me3explorer. IMO there is no reason NOT to get this puppy. It doesn't radically change the game and make geth pink and purple, it just makes the stuff thats already there look a lot better. It does take a few liberties (adding lights to some armors) but you'd have to be a real purist to really be bothered by that kind of stuff. In which case I doubt you'd be looking to mod the game anyway. 
ME2 the mods that come to mind are basically all of Jean's texture mods. There's a lot of hidden gems at the old BSN Projects page but you'd have to search for them. Like I said, they're pretty much the best definition of hidden gems just because of how difficult some can be to come across. The best bet is to search for Mass Effect and then go through all the listing it brings up. I found an awesome retexture of Kasumi's outfit for Shepard (the fancy clothes you wear during her mission) that I still use. Sadly I've never been able to find the author or the project again to thank them for its awesomeness. So yeah, hidden gems.
ME3 is a different beast. It has a lot of good stuff going for it. For example theres actually texture mods that pretty much high-res the entire game. Like, not just armor and weapons but also the locations you visit. Beautiful stuff. The problem is that getting all these mods is very time-consuming and actually installing them with ME3Explorer can be tedious and even more time-consuming. Bioware did NOT making modding these games easy. You can easily spend a few days just shifting through all the mods and getting them installed correctly. Luckily though its a one-time deal so after you do it you're set. Thats about the only plus about it though. Its up to you if you really want to spend that much time setting these mods up. I did it because I'm a huge fanboy and wanted the pretties. Otherwise I'd be happy with just a armor or weapon texture mod and be done with it.
Anyway, here is the topic where I found all those ME3 texture mods. There are a lot of other mods there too, like armor and weapons and characters. Still, imo, the environment stuff stood out the most because I don't see many of those being done nor have I seen them anywhere else but that site. Where as stuff like weapon and armor texture mods I've seen all over the place. Though it has been nearly a year since I looked into all this. Coincidentally if you really plan on modding ME3, without just using Texmod of course (easier but you have to do it everytime you launch the game) you'd need to get familiar with the website anyway since that is the forum with the guides on how to use ME3Explorer and stuff. You'll find out about that in due time if you dont already know.
Finally a great place, if only for the increased simplicity of locating mods, is the Nexus. They have a section for each game. Though from memory only the ME3 one had substantial content. Still may be worth checking them out to see some of the cool stuff they have. I got a lot of my armor/weapon/Kaidan Bearded textures from t he ME3 nexus. So much easier to browse through there then random forum sites.
http://www.nexusmods.com/masseffect/?
http://www.nexusmods.com/masseffect2/?
http://www.nexusmods.com/masseffect3/?
I should note that a lot of the armors/weapons texture mods for ME3 that you can find here on the forum (Ottemis and Smarteck) I've seen and personally nabbed from the Nexus. I couldn't say which one would be best for you, though. They're both very talented and made some fantastic armor retextures. It's personal opinion on what set you like best. I mix-and-matched some of my armors. Some from one guy, some from another. Not because the other is bad, but because tastes are different. My Shepard, however, uses a retexured black N7 Ajax Cerberus armor. Found here. For /me/ its a must have but your choice of armor and what looks better is a very personal matter.
I freakin love that armor aesthetically, though I had to fiddle and mod my game a bit to change the perks to make it go well with my class. Bless the ME3Explorer community for their guides that made that possible.
Anyway the only other mod I can think to mention is this mod that removes the relay loading screens. This requires some use of the ME3explorer program and can seem intimidating to some. Though, again, for me its a must. I only wish I found it sooner. Live an learn.
I hope you find this useful.