Upgrading the weakest link

I have recently come across a nice amount of money (yay for graduation), and now I'm ready to upgrade my computer. Atleast upgrade the weakest part of it. My specs are:

Video: ATI Radeon x1800XL
Sound: Sound Blaster Audigy
Hard Drive: 120GB 7200 RPM (http://www.newegg.com/Product/Product.asp?Item=N82E16822144414)
Motherboard: ABIT KN8 Socket 939 NVIDIA nForce4 ATX (http://www.newegg.com/Product/Product.asp?Item=N82E16813127221) (There are 2 RAM slots that are defective, but I can send it in for another one for free.)
Processor: AMD Athlon 64 3500+ (http://www.newegg.com/Product/Product.asp?Item=N82E16819103603)
Memory: 2x 512MB SDRAM

Anyways, those are the parts of my system I am willing to upgrade. What are your opinions on what part(s) could be upgraded the most and to what?

 

Note: It doesn't really matter how many RPMs your HDD can do, what determines it's speed is more the technology that the HDD connects to the mobo with. SATA is much faster than PATA.

My personal recommendation would be to return that board for the new one. Someday, you're gonna want those two extra slots, or you will be buying denser sticks, which is much more expensive than buying a higher quantity of less dense sticks.

Your sound card will probably never need upgrading, unless you're a music buff.

I would probably just give myself more RAM, if I was you. That or the video card. Gamer 4 Liff!