Uunfortunately i don’t think your going to be able to upgrade to one of those graphics card, it will require a new motherboard, possible new ram and cpu depending on what cpu skt you decide to go for, well, lots of cpu’s out there now have 64bit, hell even the intel celerons have it now!, the 3000+ is i would say a mid end cpu now, there are lots of cpu’s better than that now, the cheapest core 2 duo’s which are like £130 woop any AMD cpu.
If you ever want to upgrade your graphics card, X800XT, X850XT, 7800GS etc.. are going to be the best you can get, there pretty cheap now, but still, i wouldn’t bother buying another graphics card for AGP now unless you REALLY don’t want to upgrade you motherboard and if you want to go with the latest cpu skt then porribly upgrading the CPU, unless your motherboard already has a PCI-E slot?, if not, when you get the money, do a big upgrade all at once, cpu, mobo, ram and graphics 
in regards to what you’ve said here, I more or less agree. My socket is 754 and therefore I am limited to good cpus as from what I know 975 (i think thats it) is the better.
Secondly, I unfortunately do have an AGP gfx slot
lol and therefore again am limited on gfx card choice, however i have found some really good gfx cards such as the 7900 256mb in agp etc. i have a good psu at 500 i think it is. and yes indeed i will be doin a complete system overhaul and upgrade everything, but that wont be for another year or so (finance:() so i am trying to do the best and cheapest upgrades for now.
Also, DAMN, that monitor is Awsome!, 2ms and that vv high contrast ratio :o, what a good price too, the only letdown is that it’s not widescreen, would be alot better with widescreen now , i picked up my Acer 19" widescreen for £140 from Argos, 5ms, 700:1 contrast etc.., obviously no match compared to yours, but cheap for what it is =p.
I suppose you could defrag your Hard Drive, never knew you could defrag the Registry :s, you have got Anti Virus & Anti Spyware software installed right?
yup - thats a good monitor indeed, i was lucky to have found it and at such a good price too (i ordered it today, should be here within the next 4 days). and i agree with the wide screen thing, thats the only let down. btw, you have the exact monitor my friend has 
and yep, u can defreg registry… the program best to use is tune up utilities 2006 - Google that and visit their site and get the trial version or buy it if u like.
I have firewall, anti virus spyware adawre the works lol - so i am v well protected 