The T7400 is from Dell's professional workstation line. All of our design stations are Dell Precisions here. A couple are even the older 690 ones. Even those will spank the majority of newer consumer class pc's. The Precisions are built with a different goal - they are designed for bullet proof reliability and expansion. Very few consumer class machines can run 128gb of RAM.... Depending on the configuration of your T7400 (if it has a single processor or dual) you have either 8 or 16
DIMM slots for memory.
That being said - yes the on board SATA / SAS controllers are a generation old but regardless there is no mechanical hard drive made that can transfer at 3gb/s so it yes an SSD will make a BIG difference. No - a SATA3 SSD will not be able to run at it's maximum speed but it will still be far faster than the mechanical hard drive. We have a 250GB SSD as the boot drive in all of ours - Windows and programs run on the SSD and the mechanical drives are just for bulk storage. Photoshop will dramatically benefit with it's swap file being on the SSD.
As far as your specific machine goes - you can upgrade the processor. The T7400 uses the 5400 (Harpertown) generation Xeons - the fastest one you can run would be an X5492 which is a 3.4ghz quad core with a whopping 12mb of cache. Many are also confused on the differences between the consumer class "i" processors and the business class Xeon processors. Just as there have been a few generations of the i3/i5/i7 processors, there is a respective generation of Xeon processors that have essentially the exact same core. The Xeon's however generally have a larger on board cache, additional error correction capabilities, the ability to run multiple physical processors, etc is all. An equivalent generation i7 is no faster than an equivalent generation Xeon.... The 5400 series Xeons I guess are the same as the first generation i7 processors. To be honest though - if yours already had dual processors (potentially dual quad core Xeons) you are not going to see some huge difference going to the newest generation i7 or Xeons - sure it will be a difference but not really enough to make it worth it in my opinion.
I have steadily upgraded the Precision Workstations we have. The oldest, the 690 is now maxed out but it still is a butt kicking machine. My newest one is a T7500 (there is also a newer generation of the Precisions now as well) which I just recently upgraded to dual Xeon X5650 processors and will soon upgrade it to 96GB of ram (currently at 48gb), this one also has a 500GB SSD boot drive. The Xeon X5650 is a 2.66ghz 6 core with 12mb cache - and mine is running 2 of them so I now have 12 cores or just under 32ghz of processing. Task manager is a hoot - since these also support hyper threading Task Manager sees an additional 12 cores making it look like the system has 24 processors. :Big Laugh
Another thing with consumer class i3/i5/i7 based systems, they are single socket so they cannot run dual processors. They are however less expensive than Xeon based systems. Anyways - I ramble too much sometimes but I don't like reading mis-information. On the subject itself - me personally, I'd strongly recommend an SSD for your system.
It IS a bit tricky though on these. You have an on-board SAS Raid controller as well as an on-board SATA controller. You will need to make a couple of changes in the computers BIOS to set it to boot from the specific SATA port you connect the SSD to and due to the server class chipset you will need to set the SATA ports to IDE mode while you clone the current boot drive, then set it top AHCI mode when you are going to actually switch over to the SSD. It's not as simple of a process on these than on a consumer class machine. The best method is really to start from scratch. Just disconnect temporarily the existing drive, install the SSD, set the BIOS to boot from it (in AHCI mode), and install Windows 7 x64 from scratch, get that running, update drivers, then reconnect the mechanical drive and that will just show up as another drive with all the old data on it. Of course you still would have to re-install ALL of your programs, RIP, etc....
Chuocove also has it right.... On adding a PCIe SATA3 controller- I actually did this on my T7500 for the SSD - I bought a Syba PEX40054 card for this purpose. To upgrade to a SATA3 controller. You need a PCIe x4 or higher 2.0 slot to take advantage of this. PCIe 1.0 slots do not have the bandwidth and would see no real benefit. Your machine does have second generation 2.0 PCIe slots though so you could do this. I researched cards to do this with and for the price the Syba card is (or was 6 months ago) the best bang for your buck to do this.