It's not exactly clear what your budget is, but I would not suggest dual-core at the moment; the outlay is too much for what you get in return. Additionally you can buy a mainboard that takes dual-core and upgrade when dual-core prices fall. A single core AMD 64 3700+ San Diego will provide quite a boost in performance from a P4 2.4 system. Prices for AMD chips rise steeply after this level for lesser performance returns. 3700+ chips also overclock extremely well.
Graphics wise, I'd definitely recommend you go PCI-Express. You can always sell your Xmas present on eBay. SLI mainboards will allow use of one PCI-E card or two [GeForce] PCI-E cards to be connected in tandem for faster operation although, in my opinion, it's only of real benefit when you're installing top-end cards. Crossfire is ATI's competing system to SLI and requires a Crossfire compatible mainboard. You also need to take into account the extra heat that two fast graphics cards generate.
In fact new systems generate quite a bit of extra heat than you'll be used to. For that reason it's essential to have a case with good cooling. Try to find one with 120mm fans or at least one that can take 120mm fans, they spin at a lower speed than their 80mm counterparts and for that reason are less noisy.
Watch what CPU heatsink-fan you get. Don't get the AMD boxed retail processor; go for the OEM chip. That way you're not paying extra for a fancy box and a rubbish noisy heatsink/fan. Use the saved money on a quality CPU cooling arrangement.
If you're thinking of adding new memory to the existing memory you have, please ensure that it's the same exact memory (memory manufacturers module product code) If you try to mix memory modules, you could end up with random crashes, stability problems or your machine might not even boot at all. If you don't know what your existing memory is then I'd buy all new memory from a good quality brand like Kingston, Corsair, Geil, etc. You can also check on mainboard manufacturers' websites what memory modules they have certified for use.
Finally a word on power supply choice: get the best you can afford. Brands such as Seasonic, Enermax, Antec will sort you out. Most PSU's now come with 120mm fans for quiet operation, but Seasonic are particularly well known for quiet operation. Unfortunately the cabling isn't as generous, or well presented as some of the other competing brands.
Hope that helps ye!