Need Some Advice

Motherboard and HDD would be a complete waste of money without netting you any performance at all. There are two things I would get on this PC though, first one is a decent graphics card (I would incline for a 8800GT or GTS, unless you have a 24" screen), and the second thing would be a sound card, because I feel that, while not necessary, it is definitely a very good investment overall for your gaming experience. (I know it was for mine)

So... basically:

1.) Definitely a GOOD graphics card (Either 8800GT or GTS if running at 1680x1050 or less, if at 1920x1200 then get HD3870X2 or 9800GX2)

2.) If you have extra that you would like to invest on your computer (Like I did), a good sound card. Here's a deal I just can't get enough of seeing, $200-250 sound card + $50 headset For $130 (or $80 after rebates) on a bundle pack.

Creative 53SB000003000 7.1 Channels 24-bit 192KHz PCI Interface Elite Pro Sound Card and HS-900 Headphone Bundle - Retail

http://www.newegg.com/Product/Prod [...] 6829102018

It wouldn't be worth it seeing as SLI sees best gains when resolution is upped. I would suggest that you get an 8800GT or even an 8800GTS G92 at most. This is what I consider the best deal for a GTS on newegg, because it bundles The Witcher:

MSI NX8800GTS 512M OC GeForce 8800GTS (G92) 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card -

http://www.newegg.com/Product/Prod [...] 6814127325

I guess you get what I mean with the sound card, when I got mine I didn't believe I'd hear a true difference, but the best way to explain it is that all sounds are 'richer', it's really hard to describe what that difference was to me. Is it worth it to everyone? Not necessarily. Was it worth it to me? Definitely. As far as I understand a digital connection should be better than an analog, but I use an analog with my Logitech X-530 and I'm loving every minute of it.