Before I install and try it out, I want to know if its worth it to make the change.
Use as a live DVD first, give it a good try, if the hardware works here it will work for a permanent installation. Even better put Mint on a usb and boot off that. Your old computer will not boot off usb? Investigate the 'plop' boot disk.
First of all, WinXP is not all that bad, if you can get it back to minimal installation it is worth keeping. I have one that fits in a 8 GB partition with room to spare.
Generally linux runs a bit quicker than windows on old computers, but the spec you give is quite repectable.
your quote "Here's what I do with the computer:"
Just the same as Win but you do not have to worry about virus and trojans. The usual browsers are available Firefox, Opera, Chrome
Same as Win, LibreOffice is very comprhensive but if you want "lighter" then Abiword + Gnumeric is a good combo.
Photo/Video editting (non-professional)
Having used it for many years I much prefer Gimp to PS for graphics. Dread having to use that Adobe Bloatware. Inkscape great for vector graphics.
This is where I part company with linux. Avidemux within limitations is not bad. Same applies to Openshot, not so keen on the rest Kdenlive, Cinelerra etc.
That is why I suggest keeping WinXP, just do not let it anywhere near the internet. I can get my Haupauge card to capture and Womble to edit before I have set up either process in linux.
Watching Videos (HD if possible, I realize HD doesn't run too well on either XP or 7)
Well, VLC works much the same in linux as in Win. dont expect too much
Thunderbird if you want big, Sylpheed is not a bad little email client.
Set up 10 - 15 GB size partitions. one for the system and one for 'home' make what is left over into data partitions. If you keep WinXP make a NTFS partition so you can share data with linux.
best of luck