3D Game maker is an excellent choice for making children's games as AlanC said.
However, if you want to make first person shooter games then get FPSC.
As far as buying a new computer for FPSC X10, here is my advice for your situation...
Buy FPSC v1 and download the current update, now.
Play with it and decide for yourself if the added features of X10 will be worth the invesment of a new computer.
I think after you have tried FPSC v1, then you will want to buy the X10 version and a new computer.
Besides, you are gonna have to do the upgrade eventually anyway, and the prices of the hardware will come down a little more while we wait for the release of FPSC X10 and you play with v1.
I bought a new computer mainly because of FPSC X10. I would have waited longer to upgrade but I want to be ready when it is released. This way, I still have the old machine that can make games for XP with v1, and I'll also have the new puter for making X10 games for Vista. I am one of those people who think it will be well worth it, and having both is the best solution for me. I have 3d GameMaker also, but I use it for non-commercial stuff because the 3DGM and FPSC licenses are different. It all depends on what you want to do and why you are doing it.