Need help catching up on new hardware:

I'm looking to upgrade a few parts on a budget ranging from $400 optimal to around $900 max, but recent shopping has reminded me I don't know shit about any hardware made after like, 2000. I'm going to replace the motherboard/processor/ram, and I'm not sure about the video card. People say I shouldn't buy anything new without PCI-e, but I'm reluctant to change my videocard for some fuckexpensive new one. Also interested in knowing whether the boards with pci-e and agp are just novelties or something I should actually consider.

I have the following hardware:
     Not Still Good(Will Replace):
          Intel D850GB Motherboard
          Intel Pentium 4 1500Mhz
          512MB Generic PC800 RDRAM
     Maybe Still Good(Don't Know):
          ATI 9800Pro
     Still Good(I'm Keeping):
          Soundblaster Live! 5.1
          Generic 10/100 LAN Card

If anyone can help me figure out these new hardware problems, or even help me pick out specific parts, I'd be grateful. Thnks for reading, anyways.

Go with an Athlon 64 - great price:performance ratio there (I am impressed with my new San Diego core 3700+). Don't skimp on the motherboard - get a decent Gigabyte or Asus or something - buy well and you'll have the option of moving to a dual-core Athlon 64 at a later date if you wish. If you have a decent PSU and case you can still use those. Slap in a gig or two of quality memory and you should still come in well within your budget range. Take note that AGP is on the way out as a graphics card interface, so if you do want to keep your 9800Pro and buy an AGP socket 939 board, you might find your choice for a replacement a little limited later on.

My recent specs:

Gigabyte GA-K89 Pro m.board
Athlon 64 3700+ San Diego
1Gb Crucial CL2 RAM
GeForce 6800XT PCI-E gfx

There are shitloads of motherboards based on the ULi1695 chipset that have both AGP8x and PCI-E 16x ports, you might wanna get one of those. They are Socket 939 for Athlon 64 processors, 3000+ to 4000+ should be usable out of the box most of the time. Be careful to research which you get tho, there seems to also be shitloads of problems with a lot of those boards at first... but once you iron em out it should well run without any problems. I personally am thinking of getting one of those too for the same reasons as you, keeping my AGP 6600GT. ASRock 939Dual-SATA2 to be exact. My choise was based on it's extremely cheap price and relative ammount of problems I might encounter with it tho, I am sure there are more expensive and perhaps better ones available which might still fit into your budget. Well have fun!