Even on the native drive bays which are SATA II an SSD will make a big improvement. All you need is an adapter to fit SSD to the drive sled.
I would advise to not buy a 2008, you should be able to find a 2009 for $495 and up.
You can add SATA III PCIe cards designed for SSDs. Or there are PCIe adapters, not controllers, that let you install even faster (~2x+ what SATA III offers, faster than a pair of SSDs in RAID) using an M.2 blade SSD.
So for $75 + sled adapter you can have 128GB for the system and enjoy SSD.
For $500+ you can get an Apple or Samsung blade. And for $300 you could get a pair of 250GB SSDs and controller.
So at least 3 or 4 different options.
If you might want to do a CPU upgrade, you want the 2009 model (4,1) as a minimum. If you are sure you're OK with the stock CPU's, then the 2008 is usually cheaper to buy.
The units you are quoting appear to be MB/s as the numbers are correct for the SSDs but I wouldn't expect any better than 60-70 MB/s read/write from a 5400rpm HDD . You will get up to 150-160 MB/s with a modern 7200rpm & 100-120 MB/s with an older 7200rpm.Another big issue imho is the fact that only PCIE slots 1 and 2 are PCIE 2.0. Slots 3 and 4 are only 1.0 so you will end up playing PCIE slot sudoku trying to figure out which cards you could/should put where to maximize performance. On a 2009 or 2010 model all slots are 2.0. 2008 models are less likely to reliably boot from a PCIE-based SSD, although some people are able to do it without incident.
Also, the RAM on a 2008 is not only slower, it is also more expensive and requires special heat sinks.
Just so you have some numbers to consider - the stock 5400rpm hard drives on my 2008 were doing around 150-180 read/write. I now run two Apple/Samsung 2.5" SSDs in drive sleds 1 and 2 and they do about 250-260 read/write. I was very pleased with the improvement. You can expect PCIE-based SSD blades (discussed extensively in other threads) to deliver anywhere from 500-1500 read/write depending on what you are willing to pay and how well you do your research.