Would really be helpful if you filled in the Blanks:
http://www.tomshardware.com/forum/50797-35-read-buying-laptop
Without it is very hard to answer.
(1) Win XP support. Yes you can load WinXP on new laptops. Not really recommended. WinXP is no longer supported by uSoft. You will need to download and install all drivers for XP from manuf Website. Win 7 has better memory management. Most laptops now come with 64 bit ver of Win 7. The ONLY reason to down grade to XP is if you have some OLD programs that contain 16 Bit code.
(2) Ram. Yes you can run on 2 gigs. However; for new OS and programs the Min recommended is now 4 gigs. With 4 gigs you can run 32 bit or 64 bit OS, but with a 32 bit OS you will lose about 1/2 gig. Case in point my two older laptop cam with less than 4 gigs - My first upgrade for them was to 4 gigs of ram. Nice improvement for low cost.
(3) Visual studio - yes, Win 7 will suport.
(4) While you can run (A) core dual, (B) C2D -Core 2 Dual and any above (ie Ix-XXX or the newest Ix-xxxx (SB).
It is not always about the cpu itself, with the newer cpus you also get an IMPROVED motherboard, better power management, and newer chipsets. So here my recommendation would be Ix-xxx or the newest SB (Ix-xxxx) which also provide USB3 and improved power management. Also, unless you are also looking to gaming, you are going to be reling on the Internal GPU. With the Sandybridge (Ix-xxxx) the Internal Graphic (HD3000) is inside the CPU, not an external chipset and is very good. The Ix-xxx also used a IGP internal to the CPU - HD2000