Which is more important... plewase help me!!!

okay guys here is my situation...like 2 yeasrs back,i built my sister a comp for college... it was an amd athlon xp 1600+ with it i bought a really crap case. The one thing i noticed when i got the case (which none of the pictures showed!!!) was that the back of the case, the powersupply was switched from being horizontal to being vertical!!! THis way, the power supply covered itself right on top of the cpu.. Naturally this ****ed me off (but i didn't wanna say anything cause then my sister would have gone ballistic saying you bought me a crap case blah blah blah) so like literally the powersupply is 2 inches above the top of the cpu heatsink!!!. Well this week my sister came home, and brought her computer with her. She said the computer wouldn't start up. I lookd at it and the thing was gone! She said she left it on for like 8 days! The capacitors were all blown out... a couple of them had the inside foil showing!

Bottom line is this i gave my sister my computer (the one you see in my sig).

Now i want to get another comp (naturallhy) I want it to be a p4 (maybe an athlon 64.. if the price is right) but highly lookikng at a p4. my problem is this i only have about 400 dollars to spend! that's enough to buy me a p4 and a motherboard... and either a case, or 512mb of pc3200 ram... which should i get... currently i have a stick of pc2700 ram that i could use... and buy a new case (as i don't want to use that crap case of my sisters!!!) or should i just use that case and buy 512 mb of pc3200 for the p4???

which one should i get guys as i really don't know

thanks for your help in advance.

If you only have 400 dollars then perhaps it would be wiser to just go with an XP/NF2 setup....?

i've considered that also, but to be honest, i'm getting tired of the ol athlon xp thing... i want to try something new something better... something well different... i've built nothing but athlon systems for the past year!