what about the rx 460 low profile or rx 550 low profile?
I'd have to see the exact cards in question, but they are both a bit much for the 240W PSU to handle safely over a long time period. And if you were going to chance using that high wattage a card on that PSU, the GTX 1050 Ti would be safer and faster. But, like the above, still not recommended unless you do so at your own risk.
A low profile GT 640 DDR3 1GB will work, but it is a 65W card and will be pushing the capacity of the PSU. The GT 1030 is newer, faster, more VRAM, and is only a 30W card. Much preferred.
what about the rx 460 low profile or rx 550 low profile?
I'd have to see the exact cards in question, but they are both a bit much for the 240W PSU to handle safely over a long time period. And if you were going to chance using that high wattage a card on that PSU, the GTX 1050 Ti would be safer and faster. But, like the above, still not recommended unless you do so at your own risk.
if I change the psu for a desktop atx 300w will work?
"300W" is a bit of a relative term. Total wattage doesn't mean much for gfx cards. It is all about the +12V rail that matters. And finding a replacement 300W PSU to fit that Optiplex SFF is darn near impossible. If you have one in mind, please link me to it. I could use one for my Optiplex 790 SFF