You need to tell us your system in more detail.Mobo,psu etc
With the system you have(packard bell) it's not really worth it to upgrade.IMO.
You can't really change to AMD unless you get another mobo aswell.
Upgrading the cpu is not really worth it in your case(packard bell)
Price for changing cpu's is more dependant on where you live.
Adding another 512mb of ram probably won't improve things by much for you also.
The next best thing is going to be Dual core procs?
With more details we could advise you better.
Tt Lanfire
MSI K8N Diamond
AMD64 3000+
1gb pc3200
OCZ Powerstream 520w
6600GT
WDCaviar 160gb sata
WDCaviar 160gb sata;~}