The 300W part is combined, so you should be good getting the GT 240 (like I said I have the same 300W as you and it runs the GT 240 well), but that's about the limit in terms of performance. I know it probably says 400W or something on the site, but that's just in case the power distribution is different (gives the company some leeway). Really the only thing that matters is that there is 18A of 12V.
No the minimum requirement is listed as 300W on the Nvidia website.
I took the liberty of calculating the PC's power consumption. However, I am not entirely sure as to what components are currently installed so based on Dell's (possible) specifications for the Inspiron 530 I filled out the form on
http://extreme.outervision.com/PSUEngine and ended up with a 362-Watt consumption. (90% TDP and 20-30% capacitor aging)
In all fairness, I did select the Intel Core 2 Quad Q9650 which is unlikely to be present but if I pick the oldest Core 2 Quad in the list it's actually 372-watt.
This is including the videocard chosen by Dragon.
I still stand by my point that the PSU needs to be upgraded if the videocard is to be installed.
I guess we could do a recalculation based on her exact specifications but in my professional opinion, 400W of a decent brand is the absolute minimum for a gaming machine.