Buying a 2nd system

I want to buy a 2nd system and have to decide if it will be my main one or the 2nd one.

Got that so far? Good. Here is the current machine:

Gigabyte GA-EP35-DS3R
Q6600 CPU
GeForce 7950 GT
G.Skill DDR2 4GB
Audigy 2ZS Sound card

The only requirements are 1) need good sound 2) need a Nvidia video card

The rest, AMD or Intel, is up in the air. Can anyone recommend something? I guess it starts with the motherboard (and socket) so I am posting here.

I also have to decide if it will be DDR3 or DDR2. I can probably build a more powerful system today than what I have now, right?

I know, you are not going to answer or recommend until I provide the budget! I am flexible but have my limits (affordability!). I thought something along the lines of $400.

I have a spare 2GB of DDR2 Crucial Ballistix RAM hanging around and I have an extra 20" LG LCD monitor so I don't need monitor or possibly RAM if I go with DDR2-based motherboards. However, I could always sell the RAM if I go for 4GB of DDR2 or DDR3.

Finally, I want a Nvidia graphics cards so maybe 9800 GT to GTX 260 (budget of that is no more than $200). The CPU and mobo combined should be no more than $250. However, I still need the case so this is getting up there. But, if there is reason to boost the budget a bit, I am open to suggestions.

I have two questions, though. What performance hit is taken when having a good video card but not the fastest cpu and mobo combo? I thought there probably isn't much of one or it is irrelevant or not of any real consequence. I also thought I could upgrade the cpu later so get a decent GPU and mobo now?

Second question is, should it matter that my other system is Intel socket 775? I doubt I'd do much switching around so I could go AMD AM2+ or AM3. I could even go Intel i7 but that system probably is too expensive and overkill?

Thanks to all who reply, recommend, comment etc.!