my system is a E5200, 2gb ram, asus p5n7a-vm with GF9300 igp, sata 1tb low power hard drive and a laptop 80gb sata drive, when researching the motherboard i came across consumption numbers form a almost identical machine the only difference was it was using a less efficient 65nm cpu and not a 45nm like mine, total draw 80w under load i run mine from a 120w itx psu and have had 0 issues.
If you work under the assumption your psu is efficient you'd want 130w from it realisticly, 140 would be nice, so some research shows an i7 920, asus p6t, 6gb ddr3 a velociraptor hard drive on win7 system fitted with an hd5450 has a total system draw of 171w, i think with a little research an acceptable gpu and cpu will do fine on a 160w psu as long as the psu is efficent. this last point may be an issue if the computers are getting on and had no doubt extensive use in a school environment, look on ebay for as new replacemants, a new psu from a reputable brand, are they dells? would do the job.
