Similarly, if you buy a SATA III SSD, it will only run as fast as the SATA controller in your Dell.... meaning it will be slower than it is capable of, but faster than your HDD. But the SATA III drives are backwards compatible to SATA II and SATA I, so it should work.
My typical advice to anyone is: Don't spend money on a very old computer. Of course, there are exceptions to anything, but the decisions are yours.... whether you will use this laptop enough to justify putting money into it (or whether you might give it to a friend or family... something I do sometimes). You might test it awhile before even buying RAM to see how it goes.
Your screenshots (
here) are very helpful. They show your RAM is dual-channel DDR2 SDRAM at 667 MHz. It also shows your 2 GB is in one slot, and you have another slot for memory that is empty. You can usually find the memory in a trap door on the bottom of the laptop. Take a look to confirm these things.... two slots, and one is empty. The BIOS reports the memory as DIMM, but laptops are often more specifically SO-DIMM. Find your owner's manual... or Google up the manual for this model Dell, as
@wizardfromoz suggests, to be absolutely sure that you get the right memory. Adding another 2 GB module to the empty slot should be easy and inexpensive. Because the computer is so old, it may not be worth going for the maximum, which your manual will specify. The more RAM you have, the better... usually. But there is a point of diminishing returns too. The Dell will never be a screaming powerhouse... it's just too old for some things.
On replacing the HDD... I'd suggest getting Windows cleaned off and having you just use this for a little while. It may not be too bad, actually, and it will be a huge difference from your Windows 10 experience lately. The cheap SSD's will still be there a week or a month from now if you want to get one anyway, though prices fluctuate. The $22 SSD went up to $27, but the other is still $36. Here they are:
128 GB SSD and
256 GB SSD. You can shop all SSD's at NewEgg (
here). I ordered three of the smaller ones, but they haven't arrived yet, so I can't say whether I like them or not. You might also find good deals elsewhere, like
Amazon.
Okay, enough of my drivel about this. I'll make another post about your Windows hard drive below.