This is my current pc. I have now US500,00. I'm thinking in just buying a new GPU because the 250GTS seems to be a problem when playing some games like BT3. My motherboard is old, i know, and has only a pci express x16 1.0 slot. I was wondering a 670GTX or 7970 because GPUs are retrocompatible, but would the 1.0 bottleneck it too much? Should i just wait with this for let's say one more year untill I save more money or just go and upgrade the GPU for a brand new one and use it in my final build? I don't want to change my current motherboard because them I would need to change CPU, RAM and everything else, and i don't have money for a good build right now. And i ask this because if I'm building my next pc in one year, prices of current hardware will drop and it will be cheaper. BTW I have a 650W power supply. Thanks in advance
Your CPU should be fine, but your slow RAM and the PCI 1.0 will slow you down some. However, if you drop the money into the GPU now you will see a performance increase, and you can simply move it over to a new PC that you can build next year. The 670s won't be quite as top of the line next year, but they should still be able to handle any current games on good settings, and if need be you can buy a second one and SLI them.
tl;dr - Buy the GPU if you want for an immediate increase. Build a new computer next year when you've got the budget for it and reuse the GPU.