Retiring hardware?

I've recently been noticing a decline in performance in my once blazingly-fast computer (it's been a while though ), and was wondering if a CPU upgrade would be in order.

I'm currently running a:

Athlon 64 3500+ NewCastle (with the 130nm making OCing pretty hard...)
S-939 A8V Asus motherboard
1 GB 200mhz dual channel corsair RAM
GeForce 6800 GT

What are my upgrade options here? I've been reading around (notably http://forumz.tomshardware.com/har [...] 25217.html
which features a similar rig, except with a 8800GTX) and it seems like a new processor would be in order before socket 939s completely die out. However, would I see a more noticeable jump in performance with more RAM/better GPU? I haven't OC'd this processor at all at the moment, but would be willing to for a future processor more suited for it.

Done all that... I do have an extra drive that I might reformat, install XP and use as my new boot-- I haven't reinstalled since I first put this rig together.

Edit: I've recently stumbled upon this http://www.pcstats.com/articleview.cfm?articleID=1758 guide, but can't get results remotely close to theirs; While I am using an OEM fan, heat does not appear to be the issue; I just simply get BSODs on startup at around 214mhz FSB. Advice?