I have never tried the Ethernet over power line before, but that may be your best bet(let us know how it works). If you look into the Xbox 360 slim and Wifi problems, you are going to find a bunch. The older Xbox with the add on WiFi blows the slim out of the water with sensitivity and transmit power. I know above that it was mentioned one would think that the slim would be better, but it is not the case. I am an Electronics Engineer and do not work for D-Link, but I use their products. My youngest(18) has his friends over for gaming parties, so I have 3 slims accessing my network all at once via WiFi. I have had to jump through some major hoops so my kids can do this. I run a high powered router that has been tweaked to work with the Xbox slim, on top of that I run another A/P for the rest of my family so they can have full speed access to the internet. I need 2 WiFi networks because Microsoft can't design a descent WiFi antenna!
I either recommend cable to the Xbox slim, getting a 3rd party gaming adapter that connects to the Xbox's Ethernet port directly(a lot of routers will have a bridge mode, this works well instead o***aming adapter and you usually get 4 Ethernet ports to hook to other things) or getting a high power router/AP in the 500Mw to 1 watt range that the slim can hear. Also the AP's I chose had a better receiver on them.
As far as a repeater, lag city and a waste of good money.
When considering 1 bar versus 5, as I said above, the Xbox slim is deaf, but my youngest lived with 1 bar for quite some time before I got around to cranking up the network, the funny thing, the old Xbox 360 that went RROD on him got 5 bars of WiFi in the same place his brand new wiz bang toy was with the same router, NOTHING MOVED. Don't blame the router manufacturer for Microsofts poor engineering.
Just for comparison, my android phone will access his network from about a 1/4 mile(line of sight) and get full bars, when his slim still only gets 4 bars at 50ft.
Hello. A quick update. I finally purchased a powerline ethernet adapter from Frys yesterday to hook up my XBox 360 slim and DirecTV receiver to the internet (Wired).
I went with
http://www.frys.com/product/6558573?site=sr:SEARCH:MAIN_RSLT_PG. It comes with a 4-port gig switch and it has QOS enabled for all 4 ports. 1 port has full priority, second port has less priority than 1, the third port has less priority over port 2, and the last port has the least priority.
It was very simple to set up. I actually didn't read the directions. I read the quick-start card to make sure I was enabling the security correct tho. **Make sure to plug in every adapter in the wall directly! They won't work hooked up to a power strip.**
As of now, when I open up the Windows Media Center on my 360 and run a network test, the graph is pegged all of the way on the top!
I was SO excited, I decided to put my network to the test:
1) I was downloading a DirecTV VoD movie.
2) I was torrenting a file.
3) I was transferring a large file from my laptop to my NAS.
4) I was streaming a Blu-Ray quality movie from my NAS to my 360. (Lower quality Blu-Ray movie. Size: 5GB)
5) I had both laptops running WoW.
RESULT???
NO hiccups at ALL! The movie never pixelated/lagged at all. Both WoW laptops were reporting a latency of 98ms, and 101ms!

I also run my network on 5Ghz N. There is WAY too much interference in my area (Too many people on 2.4Ghz). I would normally get a connection of 54Mbps (or less) on N.
Running on 5Ghz N, both laptops are always pegged at 270Mbps!
Hope this information might help others that might be having an issue with their network. This product works great with the DLink, DirecTV, and XBox 360.