This is all mainly opinion mostly based on what I was reading 6-10 months ago for building my own machine... I have nothing against Dell or other well respected system builders (there aren't many IMO tho) but I prefer to build my own.
First off.... it should be an Intel / nVidia system.  That's my opinion.
http://www23.tomshardware.com/cpu_2007.html?modelx=33&amp;model1=921&amp;model2=873&amp;chart=444  - there's a CPU comparison chart there, selected price/performance sort... Intel and AMD are both pretty equal there but if you think you might want to upgrade to a quad core in the future (perhaps when they come down in price) Intel already has CPUs that will go into the same socket as your dual core motherboard.  I'll let someone else go on about the virtues of an AMD system.  I decided against it but now things might be different.

http://www23.tomshardware.com/graphics_2007.html?modelx=33&amp;model1=723&amp;model2=708&amp;chart=318 - there's a video card comparison chart set to list by overall game fps scores.

I'll blindly go with Corsair for RAM... 2GB of PC6400 should be plenty whether you run XP or Vista as the OS... but I don't run Vista.

http://www23.tomshardware.com/storage.html?modelx=33&amp;model1=136&amp;model2=274&amp;chart=48 - there's a hard drive chart.  I would stick with Western Digital and Samsung - based purely on my personal experience with the drives - I'd avoid Maxtor based on my personal experience too.  Size of the hard drive is best left up to you - I'd go with 250GB but an 80GB would be fine for just the system drive if you want to go with an extra drive for installing games to or any other special purposes.

I think 600-700 watts is going to be sufficient for a power supply but I don't know for sure off-hand.  http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&amp;N=2010320058%201131414179&amp;bop=And&amp;Order=RATING  - I just sorted by rating at newegg and went with the one at the top of the list.

Lian-Li cases are awesome... the PC-6x series has a removable motherboard tray which can help with installing and troubleshooting hardware.

Putting this all together at newegg results in a price just under $900 before shipping.  I have no idea what your options are for having computer stuff delivered or even if you are comfortable in putting everything together yourself.  This is just what I would do if I had to throw something together today with little or no time to research everything to death.  
https://secure.newegg.com/NewVersion/Wishlist/PublicWishDetail.asp?WishListNumber=6150171&amp;WishListTitle=Sauron 

The motherboard I picked has on-board sound... if I had to pick a sound card I'd probably just go with a Creative PCI-E card... add another $50 for that.
http://www.newegg.com/Product/Product.aspx?Item=N82E16829102016

I spent at least 2 months looking at reviews and whatnot when I built my last machine... and even with all the research I still hit a snag or two along the way.
https://secure.newegg.com/NewVersion/Wishlist/PublicWishDetail.asp?WishListNumber=6055267&amp;WishListTitle=Ziggo+V+4 - that was the final version of parts from newegg for my computer back in February... it came to just over $900 at that time without a case and PSU.
http://videosean.blogspot.com/2007_03_01_archive.html - go there and scroll down if you want to read my ramblings on my own build.  I posted about almost every little problem and inconvenience I had at the time.