Standard "Help !" Msg, but not really....

I'm rebuilding my wife's PC, which has an Asus P5E motherboard; I purchased 2GB of OCZ flexXLC memory, so I should be OK on that [that's replacing 2GB of KIngston memory that wasn't quite up to the task].
The motherboard has an Intel E8400 processor, and I was thinking of going with a quad if I can pick one up for possibly a little less than $300. I'm not sure which one just yet though; any suggestions appreciated.
The PC is my wife's home PC and is a general use PC, with emphasis on gaming, but not really any games that are that intense (e.g. enough to justify the purchase of a "super" video card, etc.....).
I'm just tired of her telling me how fast some Dell PC is where she works, and I know it couldn't be that fast ! But, I'm also trying to construct a nice system for her, and I'm thinking of going with water cooling, as I already have the Swiftech CPU and GPU coolers, and all the associated hardware.
I've read a little on setting this board up, but not that much...
I think I know where to get a good download to flash the bios, but from that point, I'm working/learning [you can never keep up !, or so it seems...].
and memory and processor settings are not new, but I just haven't "played" with them in a while. The case for the PC is really nice, and has a Seasonic 550W power suppy, which should be enough power for what my wife does ?. [I think; "If some's good, more's better" does apply to quality power supplies, but hopefully 550 is enough...]].
Comments, suggestions, advice, whatever....very much appreciated !!!!

I would Defenitly go with a bigger PSU the E8400 is a super fast cpu, You could allways overclock it

I'd just be interested in what is inside one of these "Super Fast Dell's" that your wife's on about? I'll bet it's not as fast as what she's already running. As Plasticjesus stated, the E8400 is a fast enough cpu.

Thanks for the replies Plasticjesus & Python15. OK, that sounds good to me if you all think the CPU is fast enough; and with water-cooling and that OCZ memory, overclocking is definitely going to happen. I really don't want to purchase another PSU, but, that could happen too; I notice that many of the units I see have 750W or more; 550W is probably minimal for a system like this IMHO.
BTW, I didn't mention the drives, which are dual 384GB Seagate SATA's; I will mirror them probably, using an LSI controller.
If I can get 3.5MHz out of the system with rock solid stability, I'm sure that will be more than enough speed.

I have heard of people getting that cpu to 4ghz

So with water cooling im sure 3.5 is undershooting