I don't think you can buy a Pentium IV 2.4 GHz and GeForce 8800/ATI Radeon X1900 new in
any country these days. Might be able to find it used, though.
mechaerik is right that the CPU is important, though. And indeed, whether it will work with the new motherboard is critical. If you have an Intel CPU, you're really out of luck for getting a 2006 Intel CPU in a new motherboard, even if the motherboard isn't the latest model. You're probably out of luck with AMD, too, but are less guaranteed to be. So what this means is you probably will need to get a new CPU in order to use the new motherboard (and it sounds like a new motherboard is necessary to use a new graphics card). Unfortunately, that means you probably would also need new RAM, since your 2006 RAM is probably DDR2, and any new motherboard would require DDR3.
The lowest-cost motherboard + cpu with integrated graphics + RAM that I can find new (in U.S. online stores) that would likely run Crusader Kings II is the
AMD A4 6300 CPU + GPU for $55, a
$50 motherboard that is compatible and has non-horrible reviews, and 2 GB of RAM for around $16, which winds up being $111. This would assume that everything else could be re-used, though, which may or may not be the case. Local prices may vary, which could be a good thing - you might get a good motherboard + cpu combo that would bring the price down a bit.
The good news is that buying used may also be an option. My 2007 laptop, which is only slightly newer than your computer, can play Crusader Kings II, albeit barely. The same model is selling on eBay for $120 - $130 shipped on average. So if the goal is playability at the lowest possible cost, you may be better off buying a used computer with sufficient power from the 2008 - 2010 era. Although avoid Intel graphics from that era - they won't be powerful enough. Buy a used computer that can play it for $175 (since you probably want something slightly more powerful than my old laptop, since its GPU is actually below the minimum specs even though it does work), sell your current one for $75, and you're at a lower total cost and probably more power than a few upgrades.
The other good news is that Europa Universalis IV has the same requirements as CKII, so if you can play one, you ought to be able to play the other. I haven't tried EUIV on my barely-able-to-play-CKII laptop, but theoretically it should work.
But if you do want to upgrade, the CPU model is really needed to know what's possible, and other specifications if possible.