While I'm not sure of many good custom PC building sites (I know they're out there, though), I can recommend a good setup for your listed needs:
Intel Core i5 or Core i7 for decoding Blu-Ray movies in HD (and great gaming performance to boot)
Intel P55 or X58 motherboard (dependant on choice of processor). Brands I recommend: Gigabyte, ASUS, MSI, DFI, Foxconn, and just straight from Intel.
6 GB DDR3 RAM for gaming and watching movies, that amount should be perfect for your needs
Hard drive space doesn't matter as much, but make sure you get at least 500 GB. I have a 750 GB myself and it's just about half full; mostly of large pictures, lots of music, and many many games.
Graphics on the other hand, is very important these days. For the most part, the faster the better. My recommendations are an ATI Radeon HD 5770, 5850, or 5870, as those are great at high-definition Blu-Ray movies and, especially the 5800 series, are superb at playing 95% of games at 1920 x 1080 or 1920 x 1200 with full settings and able to achieve good frame rates (not slow and no picture jerking in the game).
You can get a Blu-Ray player drive for under $100 now, and Blu-Ray burner drives for under $200 (some are even under $150), so keep that in mind, and these drives almost always also function as normal CD and DVD read/write drives too.
And obviously I would recommend Windows 7 64 bit to tie it all together, but I'll let you decide whether you want Home Premium or Ultimate. The ATI HD 5000 series graphics cards are the only ones that FULLY support all of the features of Windows 7 (since nVidia hasn't perfected their Windows 7 cards yet), hence the reason I recommend them.
That is my recommended build setup. If you're curious, here is my build (Granted I only game at 1680 x 1050 and I don't watch Blu-Ray movies on my computer):
AMD Phenom II X4 940 @ 3.5 GHz
8 GB DDR2 800 RAM
750 GB Hard Drive
ATI Radeon HD 4850 512 MB (soon to be an ATI HD 5850)
Creative SoundBlaster X-Fi Xtreme Audio (soon to be an X-Fi Titanium)
2 DVD burner drives, one with Labelflash, one with Lightscribe
Windows 7 Home Premium 64 bit
Here are some deals on Blu-Ray drives, mind you they all were under $150 on Black Friday. As for brands, LG and Lite-On are both good brands with good reputations, so find one in your price range from them and go for it.
Blu-Ray burner drives:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&SubCategory=600&N=2010100600&SpeTabStoreType=1
Blu-Ray reader drives (were all under $100 on Friday)
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&SubCategory=598&N=2010100598&SpeTabStoreType=1
How will your sound system be set up? Because you can get an ATI 4890 or 5800 series card for gaming, and they can output 7.1 channel HD audio over HDMI along with full 1080p picture, and then have no need for an extra sound card (as the sound would be produced within the graphics card itself). With an nVidia card, you need an extra connector to wherever you can hook up a special S/PDIF cable (whether it be from the motherboard sound or a sound card) to be able to output sound through the HDMI.
If you don't plan on using HDMI as a sound output, then go for a sound card that has both analog and digital output connectors like a Creative SoundBlaster X-Fi Titanium or Titanium Fatal1ty card.