You can't just de-solder an integrated CPU and slap another one on in it's place, for physical reasons, and because the chipset is specifically designed for the original CPU, in most laptops. Laptops with integrated CPU's are designed in such a fashion that just the task of un-soldering the CPU, resoldering another back in place is so daunting, few would attempt it. Because of this, no effort is made to make the chipset cross-compatible with other CPU's.
There have been cases with Intel, where CPU's are made that use the same socket as older CPU's, but they require a new chipset (hence defeating the purpose of backwards compatibility, *sigh*), specifically the first few Core 2 CPU's that used the LGA775 socket. So even if you could find another CPU with the same physical architecture, it's practically guaranteed the chipset would not be compatible.
Back to the OP's question, there's not much you can do. The only component that can (usually) easily be upgraded in laptops, is the RAM, which you already have a sufficient amount of. Depending on the RAM's bandwidth and timings, replacing the modules with faster ones may yield a decent speed increase, but not much.
The lucky thing is that your laptop does not have an integrated intel chipset (*shudder*). It has an ATI 5470 Mobile, which means you may be able to upgrade the video card, if it's not integrated into the motherboard. Check your laptops manual for details, if it doesn't mention upgrading/maintenance, look it up on Dell's website. If you do attempt to upgrade it, make sure you purchase a mobile edition video card, not a standard desktop version.
Regardless, laptops are generally bad choices for gaming, as the design priority is portability, not performance. Your money would be much better used to save up and buy a PC, which are generally pretty upgradable, as long as you don't buy an already outdated motherboard. Don't waste your money getting stuck with a motherboard that's already years old, it should be at the very least, compatible with last generation CPU's (i.e for intel mobo's: Core 2, AMD is still producing backwards compatible CPU's that work on AM2+ boards, so you'd be hard pressed to not be compatible with the latest CPU's)
For a budget computer, AMD CPU/motherboard's are the best way to go. Their performance is pretty close to that of Intel CPU's, but way, way cheaper. Even though I can afford a Core i7, I've been a loyal customer for AMD for years, if it wasn't for them, I wouldn't have been able to afford my first custom build. The best thing about them is the fact they do their best to make their products backwards compatible with older motherboards. AM3 CPU's are compatible with AM2+ boards, as long as the board's BIOS supports it (which just means a BIOS update if it doesn't). Intel has a nasty habit of producing a new socket for nearly every single new line of CPU's.