upgrading computer, would like some thoughts on hardware

ok im finally gonna upgrade my computer...arg..and i have about 300 to spend and im a pretty big gamer so ram and vid card is important to me obviously

heres my setup now
amd duron 1.2 ghz
256ddr pc2700
geforce 4 mx 440 64mb
ecs k7s5a MB <---im not 100% sure thats the right model it might be k7s5d is there is one that is..lol
80 gig hdd
windows xp pro

erm..i think thats about it..its got on board sound so yeah..anyway i was wondering what i should do with the 300
i want to keep my hdd and os but im willing to change the others
i was thinking of going to

512md ddr (buying another 256 stick)
then geforce fx 5600 128mb
then im 100% clueless on the motherboard and processor,
i was also wondering what your guy's thoughts on the geforce fx 5600 is..

thanks for your help. if there is anything else you need or want to know email me at URL