Christmas List (advice request)

I have a Dell that with the exception of a HD I added about a year ago, I pretty much have left completely untouched since I bought it in August '02.  It's hanging in there pretty well, as it can run Half-Life 2 with no problems, at medium detail level.  But I'm looking to get some goodies for myself for Christmas.  Here are my specs as they currently stand:

Dell Dimension 4500
P4 2.26 GHz
512MB DDR266 PC2100 ram, (already bought another stick of 512)
Windows XP Pro SP2
Maxtor 37.2GB Hd, (C:, 75% full)
Liteon DVD-ROM LTD163 (D:)
NEC CD-RW NR-9100A (E:)
Wester Digital 140GB (partitioned F,G,H:, about 20% full)
Nvidia GeForce4 MX 420 64MB (AGP)
a basically no-name 15" LCD, VGA input only
harmon/kardon 2.1 speakers+sub

I'm in the market for a video card, a game controller, and possibly a new processor.  Specifically, I'd like some advice on the video card. I'm looking for a 256MB GeForce 6800, hopefully for under $400.  My board does not have a PCI Ex slot, so it must be AGP.  I have no idea what multiple it is (AGPx2, x4, or x8) Do I need to find that out?

I'm also looking at game controllers, if anyone has good suggestions.  I like Logitech in that regard.  And what should I go for as a processor?

PS. It looks like theres a good $200 difference between 6600's and 6800's.  Which should I go with?