I think your model is very similar to my Dimension E510. I just put in a 1gb Geforce 9400gt fanless card, and I didn't have to change the power supply. I think the 9500gt and 9600gt will fit in also. Some video cards won't fit because of the processor heatsink shroud.
What type of ram do you have now? I tried some PC2-5300 memory in my computer 2 years ago and it showed up as 533 mhz in the Set Up program. As far as I know the 667 mhz memory gets slowed down to 533 mhz on your computer too. They have some 1gb modules of PC2-4200 here: http://www.computermemoryoutlet.com/Dell-Dimension-5150-memory.htm . Unless you have a 64 bit operating system it doesn't do much good to put in more than 3 gb of ram.