I don't think that a stock Dell 5100 PSU can handle a 7900 GS. I have a Dimension 8400(in sig) that has a 350W unit that might be able to handle it. The problem with Dell PSUs is the low amperage. The 7900 GS needs at least 22 amps on the +12V rail/s. Your Dell PSU maxes out at 19 amps if I'm not mistaken. Safe bet would be to buy a new one.
The RAM you chose would work, but it and everything else on that site seem incredibly over priced. Their selection of products is very limited as well. My recommendation would be to find another place to buy parts.
i would also be careful with RAM with dell's, because they are very touch and go. Kingston have dell compatible ram as do crucial (micron) and there are even success stories with corsair - but apart from them, make sure you have a good returns policy on it if you do go ahead.
I would imagine that your PSU should be able to handle the 7900gs, as i have the same PSU and it runs a 7600gt no prob, but you should be aware that the PSU may not come with the pci power pin thingy and you will terefore require an adapter