D830 really. CPU upgrade question.
What is the best CPU upgrade the i can put in my laptop without changing out the motherboard?
Your really don't have many options with older laptops like the D830. On top of this, many of the CPU's you'd want cost nearly as much as a whole new system that includes them as a factory option (if not more), making the upgrade hard to justify. However, it may be worth consuidering if you can get a compatible CPU for relatively cheap.
The D830 seems to switch between Socket M (early) and Socket P (late) based on how old your system is, generally speaking. Some use Socket M, while others use Socket P. Socket M systems are difficult to upgrade since many of the CPU's on that socket aren't worth buying, but this isn't always the case with Socket P.
To see which one you have, see when it was manufactured to get a basic idea. Once you know, check the CPU with a program like CPU-Z to check with software. If you want to be 100% certain before buying a CPU, disassemble the laptop. Socket M (mPGA 478) is usually pink and Socket P (pPGA 478) is usually white, but this can vary.
This laptop only supports 800MHZ FSB CPU's. You will need to find the best CPU you can get that fits your budget and verify it is a 800MHz variant. The service manual (Dell) can be found here; iFixit doesn't have a guide for this.