If you’re planning to use Virtual PC or make use of the XP backwards compatibility mode in Windows 7, then you’ll need to have a CPU that supports hardware virtualization. Sadly is not very clear which CPU’s offer that feature.
I found two pages with useful info, one for Intel processors and one for AMD:
Note: for XP virtualization you will also need a minimum of 2GB of RAM.
I’ve copied the essential information (including the disclaimer) for your reference pleasure 
Disclaimer: I believe this information is accurate, but it is possible that some mistakes may be present in the following tables. caused by inaccuracies in Intel’s documentation or by editing and composition errors. I urge you to do your own research before making any buying decisions. Even if a specific CPU appears to support Intel VT, make sure that the PC’s BIOS manufacturer allows this feature to be enabled.
Intel Desktop CPUs
| Core 2 Duo | |
| E4300/4400/4500/4600/4700 | NO |
| E6300/6320/6400/6420/6540/6550 | YES |
| E6600/6700/6750/6850 | YES |
| E7200/7300/7400/7500 | NO |
| E8190 | NO |
| E8200/8300/8400/8500/8600 | YES |
| Core 2 Extreme | |
| QX6700/6800/6850 | YES |
| QX9650/9770/9775 | YES |
| X6800 | YES |
| Core 2 Quad | |
| Q6600/6700 | YES |
| Q8200/8200S/8300/8400/8400S | NO |
| Q9300/9400/9400S | YES |
| Q9450/9550/9550S/9650 | YES |
| Core i7/Core i7 Extreme | |
| I7-920/940 | YES |
| I7-965 | YES |
| Pentium D/Pentium EE | |
| 805/820/830/840 | NO |
| 915/925/935/945 | NO |
| 920/930/940/950/960 | YES |
| 955/965 | YES |
| Pentium for Desktop | |
| E2140/2160/2180/2200/2220 | NO |
| E5200/5300/5400 | NO |
Intel Mobile CPUs
| Core 2 Duo Mobile | |
| L7200/7300/7400/7500 | YES |
| P7350/7450 | NO |
| P7370 | YES |
| P8400/8600/8700/9500/9600 | YES |
| SL9300/9400/9600 | YES |
| SP9300/9400/9600 | YES |
| SU9300/9400/9600 | YES |
| T5200/5250/5270/5300/5450/5470 | NO |
| T5500/5600 | YES |
| T5550/5670/5750/5800/5850/5870/5900 | NO |
| T6400/6570 | NO |
| T7100/7200/7250/7300/7400 | YES |
| T7500/7600/7700/7800 | YES |
| T8100/8300 | YES |
| T9300/9400/9500/9550/9600/9800 | YES |
| U7500/U7600 | YES |
| Core 2 Extreme Mobile | |
| QX9300 | YES |
| X7800/7900 | YES |
| X9000/9100 | YES |
| Core 2 Quad Mobile | |
| Q9000 | YES |
| Q9100 | NO |
| Core 2 Solo | |
| SU3300/3500 | YES |
| U2100/2200 | YES |
| Core Duo | |
| L2300/2400/2500 | YES |
| T2050/2250 | NO |
| T2300/2400/2500/2600/2700 | YES |
| T2300E/2350/2450 | NO |
| U2400/2500 | YES |
| Core Solo | |
| T1300/1400 | YES |
| T1350 | NO |
| U1300/1400/1500 | YES |
AMD CPUs
“With the exceptions of Sempron-branded processors and Turion K8 Rev E processors, all notebook processors shipped by AMD include AMD-V and therefore support Windows 7 XP mode."
"With the exceptions of Sempron-branded processors and pre-Rev F Athlon branded processors, all of the desktop processors shipped by AMD include AMD-V and therefore support Windows 7 in XP mode."
"Also, all AMD Opteron processors shipped by AMD from Rev F forward include AMD-V."