If you want to O/C you have a great cost/benefit chip already. I run my A64 3000+ at 9(multi) x 250(HTT)= 2250 mhz. A stock 3500+ runs at 2.2 ghz! You will probably be fine with the stock cooler you have for a small bump like that (I was but a guy gave me a FREE Zalman cooler). Yeah, people go on 2 much about CPU limiting-heard of AA or AF?
O about the monitor. I got a HP f2105 and I have been really happy with it. 21 inch widescreen in CS:Source is ungodly in WS and everything else scales to its correct ratio and looks great(the monitor will increase size of 1024x760 to fill the correct aspect ratio of monitor or you can just do a 1:1 ratio). Check out a Dell 2005fpw I've seen those going for $450 or so (some people do complain about back light leaks tho so look around and decide that's y i got the HP). I did have a KDS 19 inch. I don't think LCD monitors are for super picky people, and yet I am am fairly picky imo, anyway what I mean to say is don't totally discount a new monitor. Cuz mine put me on a whole new satisfaction level with my pc.
Also don't forget a good set of speakers! logitech z5500s are supreme (wld also support the digital sound on ur board-has optical right?), and the 5300s sound great, (I have them and they certainly won't break the bank-but no optical/digital input).
As for the Card...yeah me too I have a 6600gt and it won't drive games like I want.
What I really want to know is how the future games (next 1.5 years) will run. Is it likely a new game will pop out in the next 1.5 years that will rob me of AA and AF @1680x1050 if I go with a 7900 or a 1900? I can't decide either. If I go with NV I also don't know whether to shoot for a GTX or be happy with a GT. No sli for me tho only 1 pcie slot so not an option for later

. I would just get an XT 50mhz bump too puny for XTX esp. since I can prolly at least get 25 mhz easily. Also, will a 1900 be too loud? I remember NV got lambasted for using a loud dual slot cooler but now noone seems to give a hoot for a gaming pc; noise is now mentioned as a factor only with a HTPC which use low to midrange cards anyways.