How to put a stock dell 745 optiplex computer into a after market computer case

Thank for your response and good advice.
I got a new HPE 8 1090t with a i7 cpu and 12 GB of ram
and some of my 32 bit Xp programs won't run on it .
I have read that 32 bit Xp will only see 3 gb of ram .
I can get a dell 745 Optiplex cheap with a 3.2 Hz CPU with 3 gigs of ram.
I could be wrong but I think the dell 745's PSU won't fit well ,it's kind of
L shaped and I think it's 305 watts .I wondering if 350 watts would be ok?
I wouldn't be using it for gameing.
I know very little and I don't want to make the case smoke.
Thanks !
Jim So. City