Anybody know if the ATI Radeon HD3650 PCI-E will fit into the Inspiron 531s ? I would probably have to find a better power supply of course. If not, does anyone know any good graphics cards that would fit?

depends on the vendor of the card who made it? is it a sapphire or visiontek or what?
and you will prolly need at least 350 watts for that card....

Its a visiontek. Does the size change that much? I know i need a better power supply. I think the minimum for this card is 300W. Thankfully ive found alot of suitable SFX power supplies online.

Unless that 3650 is a half-height or low-profile card (and from look on Visiontek's site, their's isn't), it won't matter much what power supply you have as the card won't fit. Dimensions on the case for the 531s (s for slim),

Dimensions & Weight
H: 14.2 inches (36.2 cm)
W: 3.9 inches (10.0 cm)
D: 17.1 inches (43.5 cm)

Dell Inspiron 531s Desktop

Then does anyone know a good card that will fit into the inspiron 531s? Unfortunately i can't spend too much, but i don't mind looking for a better power supply.

wait i forgot you can't put more than a 250 watt in that case

No promises on whether it would require a new power supply but one of these (or a different manufacturer if you can find one),

Newegg.com - JATON Video-PX498-DLP GeForce 9500 GT 1GB 128-bit GDDR2 PCI Express 2.0 x16 Low Profile Video Card - Desktop Graphics / Video Cards

Now if you wanted to go all out and could find a PSU strong enough to run it,

Newegg.com - SPARKLE SX98GT512D3L-NM GeForce 9800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready Low Profile Video Card - Desktop Graphics / Video Cards

Another option according to the user reviews,

Newegg.com - Galaxy 96GFF6VIFCXX GeForce 9600 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 SLI Supported Low Profile Video Card - Desktop Graphics / Video Cards

Not sure what is in yours, but quite a few there with C521's that claim it is running just fine on the stock 280W PSU; take with a grain of salt, but it is possible if the PSU is of decent quality.

yea someone else wanted to upgrade one of those dells and they don;t make more than a 250 watt psu you could get a external psu or a cheap barebones kit or also you might could get a atx case for your hardware not sure about the last one though

well you can but they don't make more than 250 watt ones...... lol i have looked before