ok first im almost positive ur old ram will not work, i had a pavillion and it came with pc133 ram (so ancient) but anyways u need to check 2 make sure the pins will fit in the new board
the asus motherboard (if buying from newegg or tigerdirect) will say the board dimensions. now im not sure about the max board size for pavillions, im guessing 10in x 8in, prolly wrong, some one can correct me but hp towers arent made to be upgraded (trust me) even some hp towers that come new OEM only come with 2 available 5.25 bays, both of which are full
i suggest trashing the case, pavillions r cheap as hell anyways go to newegg and pick out a shiney new case, nice and cheap and some very sturdy 4 the price. dont buy the case 4 the psu, they suck.
as for the MB asus is great, so is SOME MSI's, ABIT, and Megabyte has some nice ones. and that 4 gig ram upgrade i believe is only for PC3200, if u want PC4200 sticks i think the support is only 3 GB, also im not so sure about the NF2 with the asus mb, im pretty sure the one ur looking at the chipset is K8T800Pro + VT8237, thats just a guess, but i think ur gunna wanna go 4 a board that has the nF3 250 or nF3 Ultra until the nF4 comes out.
and the case issue again, most cases are generic mb support, they will say form factory or mb supported, ATX, miniATX, ATX extended, flexATX, and then it will say max mb size or however they word it, i know the 1 im getting (inless i change my mind is the ASPIRE Turbo Case Blue http://www.newegg.com/app/ViewProduc...144-018&depa=1
basically because of the large side window and 5 5.25 drive bays and hd shelves and removeable mb tray...SLICK!! lol but if u look at the bottom of that page it says "Motherboard Compatibility: ATX-Up to 12"W x 13"L" so there ya go any more questions post or email me