Looking at Page 46 of this
Dell Studio 540 document the Processor support section says that it supports Intel Core2 Quad Processors with an L2 cache size of 6 MB/8 MB/12 MB.
Intel Core 2 Quad Q8200 <=== A lot of the Dell Studio 540 PCs shipped with this one
Clock Speed: 2.33 GHz
L2 Cache: 4 MB
FSB Speed: 1333 MHz
Intel Core2 Quad Q6700
Clock Speed: 2.66 GHz
L2 Cache: 8 MB
FSB Speed: 1066 MHz
Intel Core2 Quad Q6600 <=== Some of the Dell Studio 540 PCs shipped with this one
Clock Speed: 2.40 GHz
L2 Cache: 8 MB
FSB Speed: 1066 MHz
Intel Core2 Quad Q9450
Clock Speed: 2.66 GHz
L2 Cache: 12 MB
FSB Speed: 1333 MHz
Intel Core2 Quad Q9550
Clock Speed: 2.83 GHz
L2 Cache: 12 MB
FSB Speed: 1333 MHz
Intel Core2 Quad Q9650
Clock Speed: 3.00 GHz
L2 Cache: 12 MB
FSB Speed: 1333 MHz
It really isn't worth dumping money into a better CPU unless you can find and get one one for a low price. Good luck with that. All those CPUs are EOL (i.e. End-Of-Life) products.
The motherboard's SATA controller only supports SATA1 1.5 Gbps and SATA2 3.0 Gbps speeds. If you decide to get an SSD and it has a SATA3 6.0 Gbps interface you won't get the full performance out of the SSD. You may still be able to find cheaper SATA2 3.0 Gbps SSDs.