generally a 300w limitation on gpu only can go as far as a 7750 and very close to around there generally. chaning the psu basically doubles/triples the amount of $ you can spend on the gpu.(limited by how much you put into the psu and size of case)
see any increase in performance if I upgrade my Dell Inspiron 620 PSU: No.
IF your power supply was too small or failing you would see blue screen and system instability. A working power supply does not effect performance if it can deliver the right voltages (which are then further regulated by the MB, video card, etc). An undersized or failing PSU hurts stability and availability, not typically performance.
edit: And you sir are very well prepared. "backup drives ". Most people (especially with SSDs) don't think about that untill way to late. gj.
LPC Hub Controller 1 : Dell H61 LPC Controller
LPC Legacy Controller 1 : SMSC SCH5514D-NS
Audio Device : Dell Cougar Point High Definition Audio Controller
Audio Device : PC Partner AAB0h
Audio Codec : ATI (AMD) HDMI Audio
Disk Controller : Dell Cougar Point 4 port SATA IDE Controller
Disk Controller : Dell Cougar Point 2 port SATA IDE Controller
USB Controller 1 : Dell Cougar Point USB Enhanced Host Controller #2
USB Controller 2 : Dell Cougar Point USB Enhanced Host Controller #1
SMBus/i2c Controller 1 : Intel ICH SMBus
Printers and Faxes
Printer : PDFill Writer (4000x4000, Colour)
Printer : Send To Microsoft OneNote 2010 Driver (1200x1200, Colour)
Printer : Microsoft XPS Document Writer (600x600, Colour)
Fax : Microsoft Shared Fax Driver (200x200)
Printer : Canon MF4400 Series UFRII LT (600x600, USB)
Printer : Canon MF4400 Series (FAX) (200x200, USB)
Scanners and Cameras
Scanner : WIA Canon MF4400 Series (Scanner, 600x600, USB)
Go edit your post and get rid of the serial #. Edit is one of the three icons on the far right inside the box of your post.
"With my current setup, what can be improved"
Plenty of CPU
HD7770 is plenty of video unless you change your mind about heavy gaming or unless you do cadcam. (lol, guess that proves HD7770 runs with 300w PSU and 95W CPU)
8 GB plenty of memory
Sound is fine. Some people swear by external USB DACs for good headphone. Never tried any.
Samsung 830 has a good reliability reputation for SSD and is fast
Assume you have a good monitor. I went 24" 1920X1200. I like the extra vertical space of 16:10 instead of 16:9.
Have you ever tried a gaming mouse (e.g. deathadder) ? They slide nicely.
I use a receiver as a monitor platform, and to power a pair of real speakers for audio. Like it.
Not much else I can think of. My daughter wanted a tablet for her PC -- not sure what she uses it for. I have an external raid disk array for backup and movies -- a san digital unit with hardware raid 5. It's nice but not sure I'd spend the money again.